AlgoDaily

I believe that practising algorithms every day is a long-term investment in my life.

Languages

• golang, python, js

Target topics

• Binary Search
• Binary Search Tree
• Binary Tree
• N-aray Tree(Trie)
• Graph(Dijkstra, Union Find, Kruskal, Prim's, Minimum Spanning Tree, Topological Ordering...etc)
• Stack
• Queue
• Array
• Sorting
• Hash Table
• Heap
• Linked list
• Bit Operation
• Dynamic programming(Kadan's)
• Backtracking(Permutations & Combinations & Subsets...etc)
• Math
• and more...

Questions from

• leetcode
• project euler
• glassdoor
• interviews
• ...

Started on September 1st 2018

Day	Task	Type	From	remarks
1	binary search	binary search	leetcode 704	📌
2	sqrt(x)	binary search	leetcode 69
3	search in rotated sorted array	binary search	leetcode 33	📌
4	Find Peak Element	binary search	leetcode 162
5	Find Minimum in Rotated Sorted Array	binary search	leetcode 153	📌
6	Find Minimum in Rotated Sorted Array	binary search	leetcode 153	📌2 extra solutions for day5
7	Search for a Range	binary search	leetcode 34
8	Find K Closest Elements	binary search	leetcode 658
9	Closest Binary Search Tree Value	binary search, tree	leetcode 270	1st attemp 10sep2018, 2nd 20jan2019
10	Closest Binary Search Tree Value	binary search, tree	leetcode 270	revise day9 problems with legit recursive & iterative dfs
11	Breadth First Search	tree		basics: revise breadth first search
12	Search in a Sorted Array of Unknown Size	binary search	leetcode 702	golang unsupported on leetcode, do it in python instead
13	Pow(x, n)	binary search	leetcode 50	i failed to come up with the solution myself, the solution is mind blowing
14	Valid Perfect Square	binary search	leetcode 50
15	Find Smallest Letter Greater Than Target	binary search	leetcode 50
16	Lower & Upper Bound Binary Searches	binary search		my medium article
17	Intersection of Two Arrays	binary search, hashtable	leetcode 349
18	Intersection of Two Arrays II	binary search, hashtable	leetcode 350
19	Two Sum II	binary search, hashtable	leetcode 167
20	Find the Duplicate Number	binary search, hashtable	leetcode 287
21	Find Minimum in Rotated Sorted Array II	binary search	leetcode 154	📌 very hard classic question
22	Validate Binary Search Tree	binary tree	leetcode 98	📌 1st 25sep2018 2nd 18mar2019
23	Inorder Successor in BST	binary tree	leetcode 285	my O(logn) solution is unique ✌🏻, tho the suggested solution is jaw-dropping and much more terse
24	Depth First Search	binary tree
25	Binary Search Tree Iterator	BST	leetcode 173	my first solution was super slow tho it passes the OJ. The suggested solution is awesome
26	Search in a Binary Search Tree	binary tree	leetcode 700
27	Iterative inorder traversal on BST	binary tree		📌
28	Pre & post order traversal on BST	binary tree		📌
29	Insert into a Binary Search Tree	BST	leetcode 701	recursive & iterative
30	Delete a node in a BST	BST	leetcode 450	my 1st attempt was super discursive LOL
31	Delete a node in a BST	BST	leetcode 450	📌 classic approach, bottom up recursion to replace the target node with its successor
32	Pre & In & Post Order Depth First Search	tree		revision on tree traversals
33	Kth Largest Element in a Stream	BST	leetcode 703	my first attempt "Time Limit Exceeded"
34	Kth Largest Element in a Stream	BST	leetcode 703	learned what is heap and did a heap solution
35	Lowest Common Ancestor of a Binary Search Tree	BST	leetcode 235	⭐
36	Convert Sorted Array to Binary Search Tree	BST	leetcode 108
37	Balanced Binary Tree	Tree	leetcode 110
38	Contains Duplicate III	array, BST, bucket	leetcode 220	🧐 I don't understand the suggested solutions in BST and Bucket
39	Lowest Common Ancestor of a Binary Tree	tree	leetcode 226	⭐ even O(2n) leads to Time Limit Exceeded, what a draconian challenge. checkout suggestion
40	Recursive Binary Search	binary tree		to understand the logic of recursion
41	Binary Tree Preorder Traversal	binary tree
42	N-ary Tree Preorder Traversal	N-ary tree	leetcode 589
43	Binary Tree Inorder Traversal	binary tree	leetcode 94	the iterative solution is mind blowing
44	Binary Tree Postorder Traversal	binary tree	leetcode 145
45	Binary Tree Postorder Traversal	binary tree	leetcode 145	to understand the trick of the iterative solution
46	N-ary Tree Postorder Traversal	N-ary tree	leetcode 590
47	Binary Tree Level Order Traversal	binary tree	leetcode 102
48	N-ary Tree Level Order Traversal	N-ary tree	leetcode 429
49	Maximum Depth of Binary Tree	binary tree	leetcode 104
50	Maximum Depth of N-ary Tree	N-ary tree	leetcode 559
51	Symmetric Tree	binary tree	leetcode 101
52	Path Sum	binary tree	leetcode 112	iterative, recursive
53	Populating Next Right Pointers in Each Node I & II	binary tree	leetcode 116	actaully my initial solution is O(n) in complexity and use only O(1) in space, it is good enough to apply on II as well
54	Construct String from Binary Tree	binary tree	leetcode 606
55	Binary Tree Right Side View	binary tree	leetcode 199	2nd & 3rd attempt beat 100%
56	Construct Binary Tree from Inorder and Postorder Traversal	binary tree	leetcode 106
57	Construct Binary Tree from Preorder and Inorder Traversal	binary tree	leetcode 105
58	Construct Binary Tree from Preorder and Postorder Traversal	binary tree	leetcode 889	similar to day 56,57
59	Serialize and Deserilize Binary Tree
tree	leetcode 889	serialize
60	Serialize and Deserilize Binary Tree	binary tree	leetcode 297	deserialize ✌🏻 this is a HARD question, i made it finally. this concept is very important 📌
61	Serialize and Deserilize BST	binary tree	leetcode 449	😎 actually the concept of day60 is applicable here in BST
62	Serialize and Deserilize N-ary Tree	binary tree	leetcode 428	My naive approach was to use json, it actaully beats 42.29%...not bad 😂 The suggested approach is astonishing 😳
63	Encode N-ary Tree to Binary Tree	binary tree	leetcode 431	encode ✅ decode 🤔
64	Encode N-ary Tree to Binary Tree	binary tree	leetcode 431	encode ✅ decode ✅ This question is pretty hard, it spent me 2 nights but it was worth doing.
65	Implement Trie (Prefix Tree)	trie	leetcode 208	insert
66	Implement Trie (Prefix Tree)	trie	leetcode 208	search, startwith
67	Map Sum Pairs	trie	leetcode 677	i like this
68	Replace Words	string	leetcode 648	my 1st hasty solution got passed, but i think it is incorrect so i made another one
69	Add and Search Word Data Structure	string	leetcode 211	AddWord
70	Add and Search Word Data Structure	string	leetcode 211	Search
71	Implement Trie (Prefix Tree)	trie	leetcode 208	another approach: array instead of hashtable and it is faster
72	Design Search Autocomplete System	trie	leetcode 642	constructor and insert
73	Design Search Autocomplete System	trie	leetcode 642	search(incomplete)
73	Design Search Autocomplete System	trie	leetcode 642	WTF with leetcode
74	Implement Trie (Prefix Tree)	trie	leetcode 208	just do it again with python cos im gonna try autocomplete with python
75	Design Search Autocomplete System	trie	leetcode 642	do the same thing in python and i got accepted, fuck leetcode
76	Design Search Autocomplete System	trie	leetcode 642	i finally made it in golang and I BEAT 100% submissions
77	Word Search	trie	leetcode 79	my 1st attempt got TLE, it is weird cos the notion is correct
78	Word Search II	trie	leetcode 212	its actually a follow-up of day77 problem, tho my snippet can only beats 14% 😭
79	Moving Average from Data Stream	queue	leetcode 346	2 solutions
80	Design Circular Queue	queue	leetcode 622	front, rear, isEmpty, isFull, enqueue, dequeue
81	Walls and Gates	queue/tree	leetcode 286	it is actually a lot easier to implement using dfs than using queue
82	Number of Islands	queue/tree	leetcode 200	1st attempt:dfs beats 9.4%, 2nd attempt:dfs beats 100%, 3rd attempt:bfs beats 50.43%
83	Open the Lock	queue/tree	leetcode 752	very interesting question, 1st attempt beats 5%, 2nd attempt beats 85%
84	Perfect Squares	queue/tree	leetcode 279	bfs: 204ms beats 36%
85	Count Primes	dynamic programming	leetcode 204	1st attempt TLE, 2nd attempt: learned from the others, 3rd: optimization
86	Min Stack	stack	leetcode 155	⭐️
87	Valid Parentheses	stack	leetcode 20
88	Daily Temperatures	stack	leetcode 739	kinda dynamic programming approach
89	Evaluate Reverse Polish Notation	stack	leetcode 739	very direct, beats 100%
90	Clone Graph	graph	leetcode 133	⭐️ 1st graph question: iterative bfs for now, will try different approaches
91	Clone Graph	graph, stack, queue	leetcode 133	⭐️ 2nd attempt: iterative dfs, 3rd attempt: recursive dfs
92	Target Sum	graph, stack	leetcode 494	1st attempt: iterative dfs beats 57%. 2nd attempt: recursive dfs beats 27%
93	Implement Queue using Stacks	queue, stack	leetcode 232	📌 classic question. my classic solution also beats 100%
94	Implement Stack using Queues	queue, stack	leetcode 225	📌 classic question. my classic solution also beats 100%
95	Decode String	stack	leetcode 394	🤡 interesting question, beats 100% using a stack
96	Flood Fill	recursion, queue	leetcode 733	actually the question is quite similar to the one on day82. recursion beats 92%, bfs beats 11%
97	Design HashSet	hashtable	leetcode 705	very good question. 1st navie approach beats 6.9%, 2nd another naive beats 51%
98	Design HashMap	hashtable	leetcode 706	very good question. 1st navie approach beats 3.28%
99	Contains Duplicate	hashtable	leetcode 217
100	Single Number	hashtable, math, beat operation	leetcode 136	✌🏻🤓 day 100 with a 📌 classic question, 1st, 2nd beats 51%, 3rd beats 100%
101	Happy Number	hashtable	leetcode 202	very interesting question
102	Ugly Number	math, recursion	leetcode 263	1st beats 5%, 2nd beats 5%, optimal beats 100%
103	Two Sum	hashtable	leetcode 1	O(n) beats 100%, O(2n) beats 55%, O(n^2) beats 14%
104	Find the Unique Mistyped Item In an Unsorted Array	hashtable		⭐️ Basic but not easy. I was asked by a friend who works at a top startup
105	Isomorphic Strings	hashtable	leetcode 205	1st beats 1%, 2nd beats 5.88%
106	Minimum Index Sum of Two Lists	hashtable	leetcode 599	1st beats 93.33%
107	Largest product in a grid	array	euler 11
108	First Unique Character in a String	hashtable	leetcode 387	1st beats 10.32%, 2nd beats 100%
109	Contains Duplicate II	hashtable	leetcode 219	1st beats 87.34%
109	Amicable numbers	array	euler 21	2nd question today
110	Logger Rate Limiter	hashtable	leetcode 359	i think there is only one common solution
110	Group Anagrams	hashtable	leetcode 49	1st beats 96.55%, 2nd beats 89.66%
111	Coin Change	hashtable, dynamic programming	leetcode 322	⭐️ 2 years ago, I gave up. today i nailed it 💪🏻 1st beats 13.75%, 2nd beats 25.32%, 3rd beats 96.2%
112	Combination Sum IV	hashtable, tree, dynamic programming	leetcode 377	⭐️ classic DP, classic solution
112	Coin Sums	hashtable, tree, dynamic programming, backtracking	euler 31	⭐️ this question is similar to the previous one but the tricky thing is it requires DIFFERENT combinations
113	Coin Change II	hashtable, tree, dynamic programming, backtracking	leetcode 518	⭐️ this question is exactly the same with the two on day112 but leetcode is draconian in the Time Limit
114	Design Linked List	linked list	leetcode 707	📌 classic implementation of linked list operation
115	Intersection of Two Linked Lists	linked list	leetcode 160	📌 tricky linked list question, 3 approaches but only one is considered as Linked List related
115	Reverse Linked List	linked list	leetcode 206	📌 2nd approach is more linked list related
115	FizzBuzz	array	leetcode 206	1st beats 82.49%, 2nd beats 95.48%
115	Multiples of 3 and 5	array	euler 1	did it in a row becos it is similar to day115 q3 FizzBuzz
116	Linked List Cycle	linked list	leetcode 141	📌 classic question
116	Linked List Cycle II	linked list	leetcode 142	📌 classic question
117	Remove Nth Node From End of List	linked list	leetcode 19	📌 classic question, 1 classic + 2 other approaches
117	Remove Linked List Elements	linked list	leetcode 19	📌 classic question
118	Odd Even Linked List	linked list	leetcode 118	both naive and classic approach still beats 100% LOL
118	Palindrome Linked List	linked list	leetcode 234	naive beats 100% LOL
118	Valid Sudoku	hashtable	leetcode 36	very straight forward O(3n) solution beats 31.25%. But the 2 one-pass solutions also beats 31.25% 🤔
118	Permutations	backtracking, recursion		📌 basic
118	Permutations	backtracking, recursion	leetcode 46	📌 recursive beats 77.21%, iterative beats 77.14%
119	Permutations II	backtracking, recursion	leetcode 47	📌 1st attempt 7.41%
120	Permutations II	backtracking, recursion	leetcode 47	2nd day on the same question for understanding 📌 2nd attempt use hashtable which beats 100%(46.47% for python)
121	Median of Two Sorted Arrays	array	leetcode 4	1st O(nlogn), 2nd O(m+n) already beats 100%, suggested solution is hard to understand
121	Find the K th Element in 2 Sorted Arrays	array	asked by a friend	1st O(nlogn), 2nd O(m+n), suggested solution is hard to understand(see main.py)
122	Next Permutation	backtracking, recursion	leetcode 31	1st O(n) beats 100%, 2nd O(n) is terse and it beats 100%
122	Combinations	backtracking, recursion	leetcode 77	📌 1st beats 1.22%, 2nd beats 9.76%, 3rd beats 12.20%
123	Subsets	backtracking, recursion	leetcode 78	📌 both 1st, 2nd & 3rd beat 100 but python vers beat 35.29%, 3.67% and 35.29% only 🤔
123	Subsets II	backtracking, recursion	leetcode 90	📌 1st beats 100% but python ver beat 41.22% only 🤔
124	Subsets II	backtracking, recursion	leetcode 90	📌 spent more time to understand the iterative approach, although it eventaully beats merely 29.41%
125	Letter Combinations of a Phone Number	recursion	interview	asked by the facebook interviewer
126	Recursion	recursion		📌 a little intro of recursion of Standford CS106B
127	Combination Sum	hashtable, dynamic programming, backtracking	leetcode 39	📌 similar to day113, Coin Change II. 1st beats 23.85%, 2nd beats 63.08%
127	Combination Sum II	recursion, backtracking	leetcode 40	very similer to day119, Permutations II. 1st beats 38.89%, 2nd beats 100%. 📌 Be careful of the nuance between it and Combination Sum
127	Combination Sum III	recursion, backtracking	leetcode 216	📌 very similer to day112, Combination Sum IV and day124, Subsets II. 1st beats 100%
128	Fibonacci	recursion, dynamic programming	leetcode 509	📌 classic DP. top-down recursion beats 100%. bottom-up iteration beats 100%
128	Climbing Staris	recursion, dynamic programming	leetcode 70	📌 classic DP. top-down recursion beats 100%. bottom-up iteration beats 100%
128	Min Cost Climbing Staris	dynamic programming	leetcode 746	📌 classic DP. bruce forece recursion TLE. bottom-up iteration beats 100%
128	First Missing Positive	array	leetcode 41	1 in go beats 100%, 2 in python
129	Meeting Rooms	sort, greedy	leetcode 252	⭐️interval, 1st beats 100% 🎉takeaway: sort.Slice()
129	Meeting Rooms II	sort, greedy	leetcode 253	⭐️interval, 1st beats 0%, 2nd beats 96.97%
130	Reverse Integer	array	leetcode 7	📌 1st & 2nd approach 4ms. i gave up 2 years ago, now i finally know how to do it
130	Merge Sorted Array	sort, array	leetcode 88	📌 1st naive approach 0ms. 2nd is learned from others
131	Longest Common Substring	array, dynamic programming		📌 1st is naive. 2nd approach is classic
131	Search a 2D Matrix	binary search	leetcode 74	revise binary search, both 1st and 2nd is 8ms
131	Search a 2D Matrix II	binary search	leetcode 240	revise binary search, 1st and 2nd are 32ms beats 100%
132	Longest Common Subsequence	array, dynamic programming		📌 1st dynamic programming. 2nd recursive, 3rd recursive memorization
132	Range Sum Query - Immutable	array, dynamic programming		1st brute force beats 46%, 2nd cache beats 100%
132	Range Sum Query 2D - Immutable	array, dynamic programming		1st beats 80%
133	Bubble Sort	sort		revise sorting
133	Selection Sort	sort		revise sorting
133	Insertion Sort	sort		revise sorting
133	Merge Sort	sort		revise sorting
133	Quick Sort	sort		revise sorting. spaced & in-place
134	Rectangle Overlap	math	leetode 836	i hate this kind of questions
134	Rectangle Area	math	leetode 223	i hate this kind of questions
134	Minimum Absolute Difference in BST	BST	leetode 530
134	Find Bottom Left Tree Value	tree	leetode 513	both bfs & dfs run 12ms and beat 100%
134	Path Sum III	tree	leetode 437	1st LTE. 2nd 24ms beats 28.57%
135	Find Pivot Index	array, math	leetode 724	i failed to come up with a correct approach, learned from others
135	Largest Number At Least Twice of Others	array, sort	leetode 747	1st O(nlogn), 2nd O(n)
135	Plus One	array	leetode 66	should be just one solution
135	Find the Invalid Node in a BST	array	glassdoor
135	Minimum Distance Between BST Nodes	BST	leetode 783	this question is exactly the same wih leetcode 530
135	Unique Email Addresses	string	leetode 929
135	Continuous Subarray Sum	array	leetode 523	1st 104ms beats 0% LOL, 2nd 92ms beats 33.33%
135	String to Integer (atoi)	array	leetode 8	1st & 2nd 4ms beats 100%, fuck the corner cases
136	Diagonal Traverse	array	leetode 498	1st beats 22.22%
136	Spiral Matrix	array	leetode 54	1st, 2nd beats 100% ⭐️ 3rd is stunning
136	Pascals Triangle	array	leetode 118	1st beats 100%
136	Pascals Triangle II	array	leetode 119	1st, 2nd beats 100%
136	Minimum Path Sum	array, dynamic programming	leetode 64	1st 220ms beats 0%, 📌 2nd beats 100%
137	Jewels and Stones	hashtable	leetode 771	1st hashtable 0ms beats 100%
137	Number of 1 Bits	bit op	leetode 191	📌 the key here is to practice bit operation, i ignore any attempts using strings
137	Reverse Bits	bit op	leetode 190	📌 the key here is to practice bit operation, i ignore any attempts using strings
138	LRU Cache	hashtable, linked list	leetode 146	📌📌📌
138	Correct Flights Route	array	interview	🤔 interesting...1st O(n^2), 2nd O(n)
138	Length of Last Word	array	leetcode 58	not a good question
139	Count and Say	array	leetcode 38	⭐️
139	LFU Cache	hashtable, linked list	leetode 460	📌📌📌 1st attempt learned from others, 2nd attempt is my own and new way to implement it(although it is not efficient)
140	Count Complete Tree Nodes	binary tree	leetode 222	1st 24ms beats 92%, 2nd 20ms beats 100%
141	Container With Most Water	array	leetode 11	1st 368ms beats 34%, 2nd 12ms beats 100%
141	Add Binary	array, string	leetode 67	1st 0ms beats 100%
141	Implement strStr()	array, string	leetode 28	1st 544ms beats 2.04%, 2nd 0ms beats 100%
141	Longest Common Prefix	array, string	leetode 14	1st 4ms beats 12.90%, 2nd 0ms beats 100%
141	Remove Duplicates from Sorted Array	array, string	leetode 14	1st 220ms beats 2.04%(.py 19% .js 47%), 2nd 60ms beats 100%
141	Remove Duplicates from Sorted Array II	array, string	leetode 80	1st, 2nd 8ms beats 100%
142	Spiral Matrix II	array, string	leetode 59	1st 0ms beats 100%
142	Merge Two Binary Trees	tree	leetode 617	1st 40ms beats 59.15%, 2nd 36ms beats 100%
143	Reverse String	string	leetode 344
143	Array Partition I	array	leetode 561	1st beats 5.48%, 2nd 128ms beats 12.33%
143	Remove Element	array	leetode 27	1st beats 100%. i also implement it in JS with 2 ways
143	Minimum Size Subarray Sum	array	leetode 209	️⭐️ 1st beats 5.8%, 2nd beats 20%, 3rd learned from others
143	Maximum Product of Three Numbers	array	leetode 628	⭐️ 1st beats 28.57%, 2nd 44ms beats 100%
143	Reverse Only Letters	array	leetode 917
143	Flipping an Image	array	leetode 832
144	Most Common Word	string	leetode 819	takeaway: python=>re.split(r'\W+', paragraph). go=>regexp.MustCompile([!?',;. ]). js=>paragraph.split(/[!?',;. ]/)
144	Subtree of Another Tree	tree	leetode 572	1st 24ms beats 100%
144	Subtree with Maximum Average	tree	1point3acres
144	Divide Two Integers	math	leetcode 29	📌 classic question in hk interviews
144	Product of Array Except Self	math	leetcode 238	using / got beting 60%. naive got TLE, proper approach 2836ms beats 5.92%
145	K Closest Points to Origin	sort, heap	leetcode 973	📌 1st beats 72.46%. 2nd beats 98.95% in python
145	Gray Code	array, bit op	leetcode 89	🤔 interesting
145	Longest Substring with At Most Two Distinct Characters	array, hashtable	leetcode 159	1st, 2nd beat 17.65%
145	Maximize Distance to Closest Person	array	leetcode 849	1st beats 100%
146	Max Stack	stack	leetcode 716	very similar to Day 86: Min Stack
146	Middle of the Linked List	linked list	leetcode 876	📌
146	Insert into a Cyclic Sorted List	linked list	leetcode 708	📌
146	Merge Two Sorted Lists	linked list	leetcode 21	📌
146	Merge k Sorted Lists	linked list	leetcode 23	📌 naive approach runs faster than the decent approach 😂
146	Remove Duplicates from Sorted List II	linked list	leetcode 82	📌 1st, 2nd beats 100%
147	Network Delay Time	graph	leetcode 743	📌 Dijkstra's Algorithm. Altenative: DFS
147	Cheapest Flights Within K Stops	graph	leetcode 787	BTS: TLE. need to use 📌 Dijkstra's variation
148	Longest Palindromic Substring	string	leetcode 5	⭐️
148	Maximum Size Subarray Sum Equals k	array	leetcode 325	1st appraoach LTE, 2nd approach learned from others which beats 100%
148	Subarray Sum Equals K	array	leetcode 560	1st appraoach refers to leetcode 325, therefore i came up with the idea immediately which beats 100%
149	Maze	array	glassdoor	️⭐️
149	2 Sum Closest	array	glassdoor	⭐️
149	Reverse 2nd Half of a Linked List	linked list	glassdoor	📌
149	Reverse Linked List II	linked list	leetcode 92	️📌
149	Window Sum	array	glassdoor	⭐️
149	To Lower Case	array	leetcode 709	️
149	Sort Array By Parity	array	leetcode 905	️
149	Sort Array By Parity II	array	leetcode 922	️
149	Robot Return to Origin	string	leetcode 657	️1st beats 100%, 2nd beats 0% 😂
149	Find Anagram Mappings	array	leetcode 760	️
149	Uncommon Words from Two Sentences	hashtable	leetcode 884	️
149	Rotate String	string	leetcode 796	️
150	Find Leaves of Binary Tree	tree	leetcode 336	️1st O(h*n) beats 100%
150	Second Minimum Node In a Binary Tree	tree	leetcode 671	️1st, 2nd O(n) beats 100%
150	Two Sum IV - Input is a BST	tree	leetcode 653	️1st, 2nd O(n) beats 50% only 🤔
150	Increasing Order Search Tree	tree	leetcode 897	️1st O(n) beats 100%
150	Reverse Words in a String II	array	leetcode 186	️1st O(n) beats 100%
150	Compare Version Numbers	array	leetcode 165	️1st O(n) beats 100%
150	Sum Root to Leaf Numbers	tree	leetcode 129	️1st, 2nd O(n) beats 100%
150	Number of Connected Components in an Undirected Graph	hashtable, graph	leetcode 323	️1st beats 100%
150	Maximum Binary Tree	tree	leetcode 654	️1st beats 0%. 2nd beats 100%
151	Rotate Function	tree	leetcode 396	️1st O(n^2) beats 100%, 2nd O(n) beats 100%
151	Rotate List	linked list	leetcode 61	️1st O(n) beats 100%
151	Arithmetic Slices	array	leetcode 413	️1st O(n) beats 100%
152	Round Robin	queue	glassdoor	️📌📌📌
152	Sliding Window Median	array	leetcode 480	⭐️ 1st O(n*nlogn) 100%
152	Permutation in String	hashtable, array, string	leetcode 480	⭐️ 1st O(n^2) hashtable beats 0%, 2nd O(n^2) [26]int{} beats 40%
152	Set Matrix Zeroes	hashtable, array	leetcode 73	1st O(n) beats 52%
152	Top K Frequent Elements	hashtable, bucket sort	leetcode 347	1st O(nlogn) beats 64%. 2nd O(n) beats 64%. takeaway: use bucket sort for frequency questions, 3rd ~O(n) quick select
153	Top K Frequent Words	hashtable, bucket sort	leetcode 692	1st O(2nlogn) beats 25%. 2nd O(nlogn) beats 100%. takeaway: sort things with priority queue using priority WITH other params
153	Sort Characters By Frequency	hashtable, bucket sort	leetcode 451	1st O(n) beats 66%
153	Zigzag Iterator	array	leetcode 281	1st O(k) beats 61%. leetcode doesn't support golang, did it in python
153	Triangle	dfs, dynamic programming	leetcode 120	📌 1st TLE, 2nd MLE. 3rd dp in O(n) beats 100%
154	Products' Average Rating	array	glassdoor	⭐️
154	Greatest Common Divisor	math	glassdoor	⭐️ takeaway: Euclidian Algorithm
154	Least Common Multiple	math	glassdoor	⭐️ takeaway: A * B = LCM * GCD
154	Cells Mutation	array	glassdoor	⭐️
154	Amplitude of a Tree	tree	glassdoor	⭐️
155	Union Find	graph, union find	Study Union Find	📌 Quick Find 👎🏻 -> Quick Union 🤔 -> Union Find 🎉
155	Minimum Spanning Tree	graph, union find	Study MST	📌 heap + list of hashtables 🤔 -> Kruskal(Union Find) 🎉 or Prim(heap + hashtable)
156	Go Through all the Warehouses with Minimum Cost	graph, union find	glassdoor	📌 real life problem <- Kruskal, Prim
156	Minimum Window Substring	2pointers, hashtable	leetcode 76	1st O(n) beats 20%
157	LRU Cache Miss Count	linked list, hashtable	glassdoor	1st O(n^2), 2nd O(n)
157	Four Integers	array	glassdoor	O(nlogn)
157	Rotate a Matrix	array	glassdoor	1st space O(n), 📌 i2nd space O(1) in-place
157	Rotate Image	array	leetcode 48	📌 inplace
157	Copy List with Random Pointer	linked list	leetcode 138	⭐️
158	Path Sum IV	linked list	leetcode 138	1st O(4n)
158	Binary Tree Vertical Order Traversal	tree	leetcode 314	1st recursive, 2nd iterative, both O(n)
158	Course Schedule	graph	leetcode 207	1st LTE, 📌 2nd Classic Approach: Topological Ordering
159	Topological Ordering	graph	study	📌 1st: Topological Ordering with DFS 📌 2nd: Topological Ordering with BFS/Khan's
159	Course Schedule II	graph	leetcode 210	📌 DFS, BFS
160	Order Dependency	graph	glassdoor	📌 DFS, BFS
160	Graph Valid Tree	graph, union find	leetcode 261	📌 1st union find O(nlogn) beats 100%
160	Merge Strings	array	glassdoor	⭐️ very similar to ZigZag iterator
160	Find Distinct Substrings with Exactly K Distinct Characters	array		📌1st O(n^3), 2nd O(n^2)
161	Reorder Log Files	array	leetcode 937	1st O(nlogn)
161	Dijkstra's Algorithm on a Directed Graph	graph	study	📌
161	Dijkstra's Algorithm on an Undirected Graph	graph	study	📌
161	Find All Anagrams in a String	string	leetcode 438	1st, 2nd O(kn)
162	Optimal Flights	binary search	interview	📌1st O(n^2), 2nd O(nlogn)
162	Max Area of Island	array	leetcode 695	1st O(n)
162	Find K Pairs with Smallest Sums	array	leetcode 373	1st 2nd O(nlogn)
163	Number of Distinct Islands	array	leetcode 694	1st O(nk)
163	Kth Largest Element in an Array	array, heap	leetcode 215	1st, 2nd O(nlogn)
163	Number of Islands II	array	leetcode 305	1st O(n^2) beats 6% LOL
163	Redundant Connection	union find	leetcode 684	O(nlogn)
164	Friends Circles	union find	leetcode 547	O(nlogn)
164	Sentence Similarity II	union find	leetcode 737	O(nlogn)
165	Sentence Similarity	hashtable	leetcode 734	⭐O(n)
166	Heap	heap	study	📌 implement a heap with push, pop and heapify
166	Heap Sort	heap, sort	study	📌 implement heapsort using a heap
166	Generate Parentheses	backtracking	study	📌1st O(2^2n) beats 5%
166	Count Univalue Subtrees	tree	leetcode 250	📌1st O(n) 100%
167	Decode Ways	dynamic programing	leetcode 91	⭐️1st brute force TLE, 2nd memorized brute force O(n) beats 100%
167	Unique Paths	dynamic programing	leetcode 62	📌classic DP, bottom up recursion with memorization, O(mn) beats 100%
167	Merge Intervals	sort, greedy	leetcode 56	⭐️interval
168	Non-overlapping Intervals	sort, greedy	leetcode 435	⭐️interval
168	Minimum Number of Arrows to Burst Balloons	sort, greedy	leetcode 452	⭐️interval
168	Largest BST Subtree	BST	leetcode 333	1st O(n^2)
168	Range Sum of BST	BST	leetcode 938	1st recursive, 2nd iterative, both O(n) beat 98%
169	Delete a node in a BST	BST	leetcode 450	📌[revise day30] 2 classic approaches, bottom up recursion to replace the target node with its predecessor or successor
169	Inorder Successor in BST	BST	leetcode 285	📌 [revise day23] iterative and recursive O(logn)
169	Inorder Successor in BST II	BST	leetcode 510	⭐️ interesting, would be a potential follow-up of leetcode 285
169	Closest Binary Search Tree Value II	BST	leetcode 272	⭐ 1st O(nlogn)
170	Split BST	BST	leetcode 776	📌📌📌 super hard recursion question, must revise again
170	Unique Word Abbreviation	hashtable	leetcode 288
170	Longest Substring Without Repeating Characters	hashtable	leetcode 3	📌 1st O(n^2), 2nd O(n)
170	Repeated DNA Sequences	hashtable	leetcode 187	1st O(n)
170	Find Duplicate Subtrees	hashtable, tree	leetcode 652	1st O(n)
170	Group Shifted Strings	hashtable	leetcode 249	O(n)
171	K-th Symbol in Grammar	hashtable	leetcode 779	1st LTE. 📌2nd tricky recursion. see ./idea.jpeg
171	3Sum	hashtable, 2pointers	leetcode 779	📌1st hashtable O(n^2) beats 18%. 📌2nd hashtable+2pointers O(n^2) beats 27%, 📌3rd hashtable+2pointers O(n^2) beats 54%
171	Two Sum III - Data structure design	hashtable	leetcode 170	📌1st hashtable O(n) beats 100%
171	4Sum	hashtable	leetcode 18	📌1st hashtable O(n^3) beats 15%, 2nd hashtable+2pointer O(n^3) beats 95%
171	4Sum II	hashtable	leetcode 454	📌1st O(n^3), 2nd O(n^2)
172	Insert Delete GetRandom O(1)	hashtable	leetcode 380	1st random O(n) beats 6%, 📌2nd random O(1) beats 100%
172	Insert Delete GetRandom O(1) - Duplicates allowed	hashtable	leetcode 381	1st random O(n) beats 6%
173	Find Median from Data Stream	heap, bst	leetcode 295	1st heap, 2nd BST both LTE, 📌3rd: 2 heaps
173	Lonely Pixel I	hashtable	leetcode 531	1st, 2nd beats 100%
173	Minimum Add/Delete to Make Parentheses Valid	stack	leetcode 921	1st O(n) beats 100%
173	Largest Permutation	permutation	geeksforgeeks
174	Integer to English Words	array	leetcode 273	⭐️tedious but frequently asked
174	Remove duplicates from a string in O(1) space	hashtable, bucket	geeksforgeeks	⭐ 1st O(n) space, 2nd O(1) space
175	Design Hit Counter	hashtable, array	leetcode 362	⭐ 1st python OrderedDict O(1) for all operations beats 100%, 2nd hashtable+array beats 100%
175	Design Tic-Tac-Toe	array	leetcode 348	1st O(n^2), 😱2nd O(1)
175	Valid Tic-Tac-Toe State	array	leetcode 794	my thought is so stupid
176	Repeated Substring Pattern	array	leetcode 459	1st O(n^2), 2nd O(n)
176	Repeated String Match	array	leetcode 686	1st O(kn)
176	License Key Formatting	array, stack	leetcode 482	1st O(n)
176	Serialize and Deserialize array of string	array	geeksforgeeks	O(n)
177	Letter Case Permutation	permutation	leetcode 784	1st permutation+hashset O(2^n), 2nd permutation O(2^n)
177	Best Time to Buy and Sell Stock	dynamic programming	leetcode 121	📌📌📌1st dp O(n), 2nd heap O(nlogn)
177	Best Time to Buy and Sell Stock II	dynamic programming	leetcode 122	📌📌📌1st, 2nd dp O(n)
178	Best Time to Buy and Sell Stock III	dynamic programming	leetcode 122	📌📌📌1st O(n^2) LTE, 2nd O(2n) beats 32%
178	Minimum Time Difference	sort	leetcode 539	1st O(nlogn) beats 73%, 2nd O(n) beats 93%
178	Next Closest Time	string	leetcode 681	⭐️ so many corner cases, it was the worst attempt i ever made
179	Game of Life	array, bit op	leetcode 289	1st space O(mn), 2nd space O(1)
179	Add Two Numbers	linked list	leetcode 2	📌1st, 2nd O(n)
179	Add Two Numbers II	linked list	leetcode 445	📌1st O(n)
179	Topological Ordering Of IDs	graph	revise	implement topological ordering of a list of IDs
179	Alien Dictionary	array, graph	leetcode 269	📌1st BFS, 2nd DFS
180	Letter Combinations of a Phone Number	recursion	leetcode 17	📌
180	Multiply Strings	array	leetcode 43	📌
180	Add Strings	array	leetcode 415	📌
181	Valid Word Square	array	leetcode 422
181	Maximum Subarray	array	leetcode 53	1st O(n^2) 📌📌📌2nd, 3rd O(n) Kadan's Algorithm. This problem can be applied to Minimum Subarray
181	Maximum Product Subarray	array	leetcode 152	1st O(n^2) 📌📌📌2nd O(n) Kadan's Algorithm. This problem can be applied to Minimum Product Subarray
181	Subarray Product Less Than K	array	leetcode 713	⭐️sliding window
182	Word Ladder	bfs, hashtable	leetcode 127	1st LTE, ⭐️2nd O(n^26*l)
182	Word Ladder II	bfs, hashtable	leetcode 127	LTE, revise later
182	Minimum Genetic Mutation	bfs, hashtable	leetcode 433	1st O(MN)
183	Bold Words in String	hashtable, sort	leetcode 433	️️⭐️1st O(nlogn): interval problem
183	Add Bold Tag in String	hashtable, sort	leetcode 616	⭐️1st O(nlogn): interval problem
183	Number of Segments in a String	string	leetcode 616	takeawaystrings.Fields(s)
183	Sort Colors	bucket	leetcode 75	0th merge sort O(nlogn), 1st bucket sort O(2n), 📌2nd moving zeros swap O(2n), 📌3rd partitioning swap O(n)
183	Move Zeroes	array	leetcode 75	⭐️swap
183	Missing Number	bucket, math	leetcode 268	1st math O(n), 2nd bucket O(2n)
184	Swap Nodes in Pairs	linked list	leetcode 24	📌
184	Reverse Nodes in k-Group	linked list	leetcode 25	📌
184	Sort List	linked list, sort	leetcode 148	📌merge sort
184	Peeking Iterator	design	leetcode 284	leetcode doesnt support golang
184	One Edit Distance	array	leetcode 161	1st O(3n), 2nd O(n)
184	Zero/K Sum Subarray(s)	array	glassdoor	📌similar to leetcode 325 and leetcode 560
185	Edit Distance	dynamic programming	leetcode 72	📌📌📌 similar to LCS
185	Shortest Unsorted Continuous Subarray	sort, array	leetcode 581	1st O(nlogn), 2nd O(n)
185	Maximum Average Subarray	dynamic programming	glassdoor	📌1st O(nlogn), 2nd O(n)
185	Maximum Average Subarray I	array	leetcode 643	sliding window O(n)
185	Subdomain Visit Count	string	leetcode 811	1st O(n)
185	Trapping Rain Water	dynamic programming	leetcode 42	1st O(3n)
186	Unique Binary Search Trees	dynamic programming	leetcode 96	⭐️Catalan Number Sequence very similar to the Fibonacci Sequence
187	Valid Palindrome	2pointers	leetcode 125	📌
187	Valid Palindrome II	2pointers	leetcode 680	📌1st O(n), 2nd O(n) too but more readable
188	Cut Off Trees for Golf Event	heap, sort	leetcode 675	📌1st heap, 2nd sort
188	Lowest Common Ancestor of a N-ary Tree	tree	glassdoor	⭐️
188	Distance Between 2 Values in a BST	tree	glassdoor	⭐️build BST => find LCA => ans = depth(a) + depth(b) - 2*depth(lca)
189	Reverse Words in a String	string	leetcode 151	1st time=space=O(n)
189	Max Consecutive Ones	array	leetcode 485	O(n)
189	Max Consecutive Ones II	array	leetcode 487	O(2n)
189	Max Consecutive Ones III	array	leetcode 1004	1st O(n^2), 2nd O(2n)
189	Pacific Atlantic Water Flow	graph	leetcode 417	1st O(n^2)
190	Design Compressed String Iterator	string, queue	leetcode 604	1st O(n) unicode.IsDigit()
190	Basic Calculator II	string, stack	leetcode 227	1st O(2n), takeaway: isdigit() in python, unicode.IsDigit() in golang, and isinstance(a, int) in python for type checking, 📌2nd O(2n) too but more concise but be careful of float division
191	Array Partition by Occurence	hashtable	interview	⭐️ O(n^2)
192	Partition Labels	hashtable	leetcode 763	️️️⭐️WTF, I was asked about it yestarday during an interview. 1st O(n^2). 2nd O(n)
192	Basic Calculator	string, stack	leetcode 224	1st O(2n) tedious operation beats 25%, 2nd concise O(2n) beats 90%
193	Basic Calculator III	string, stack, recursion	leetcode 772	1st O(n^2) recursion, 2nd O(n) recursion
193	Sort Transformed Array	2pointers	leetcode 360	1st O(n)
194	Quick Select	sort	revision	revise Quick Sort and use it to find the k-th smallest element in O(n) time
195	01 Matrix	bfs, dynamic programming	leetcode 542	1st BFS O(n^2) LTE, 2nd DP O(2n) beats 96%
195	Recover Binary Search Tree	tree	leetcode 99	1st inroder+sort O(nlogn), 2nd inorder O(n)
195	Baseball Game	stack	leetcode 682	1st O(n)
195	Sort a Stack using a Stack	stack	Sort Stacks	⭐️
196	N-Queens	backtracking	leetcode 51	📌1st classic backtracking approach
196	N-Queens II	backtracking	leetcode 52	📌1st classic backtracking approach
196	Search in Rotated Sorted Array II	binary search	leetcode 81	📌classic binary search question similar to leetcode 33
196	Kth Smallest Element in a Sorted Matrix	heap, binary search	leetcode 378	⭐️ 1st O(nlogn) sort, ⭐️2nd O(nlogn) max heap, 3rd O(logn*logn) binary search
197	Maximal Square	dynamic programming	leetcode 221	📌dynamic programming in linear time O(n)
197	Binary Search Variations	binary search	revision	📌common, lower & upper bound in both iterative and recursive implementation
197	Factorial Trailing Zeroes	math	leetcode 172	⭐
198	Quick Select	sort	revision	wrote the quick select again in both go & python for better understanding