Problem 15 (#25)

* add initial template

* add solution and resources

* update readme

Author: curtisbarnard

README.md

@@ -38,7 +38,7 @@ As I work through this list I figure I would make a GitHub repo with my solution
 27. [01 Matrix #542](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/01-matrix-542.md)
 28. [K Closest Points to Origin #973](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/k-closest-origin-973.md)
 29. [Longest Substring Without Repeating Characters #3](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/longest-substring-3.md)
-30. 3Sum #15
+30. [3Sum #15](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/3sum-15.md)
 31. Binary Tree Level Order Traversal #102
 32. Clone Graph #133
 33. Evaluate Reverse Polish Notation #150
@@ -104,6 +104,7 @@ In order to practice with similar data structures I'll be placing each problem i
 - [K Closest Points to Origin #973](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/k-closest-origin-973.md)
 - [Insert Interval #57](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/insert-interval-57.md)
 - [01 Matrix #542](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/01-matrix-542.md)
+- [3Sum #15](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/3sum-15.md)
 
 ### Queue
 
@@ -140,6 +141,7 @@ In order to practice with similar data structures I'll be placing each problem i
 - [Majority Element #169](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/majority-element-169.md)
 - [Contains Duplicate #217](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/contains-duplicate-217.md)
 - [Longest Substring Without Repeating Characters #3](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/longest-substring-3.md)
+- [3Sum #15](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/3sum-15.md)
 
 ### Binary Tree
 
@@ -165,6 +167,7 @@ Within the problems above there are several patterns that often occur. I plan to
 - [Reverse Linked List #206](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/reverse-linked-list-206.md)
 - [Middle of the Linked List #876](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/middle-linked-list-876.md)
 - [Contains Duplicate #217](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/contains-duplicate-217.md)
+- [3Sum #15](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/3sum-15.md)
 
 ### Sliding Window
 

medium/3Sum-15.md

@@ -0,0 +1,122 @@
+# 3Sum
+
+Page on leetcode: https://leetcode.com/problems/3sum/
+
+## Problem Statement
+
+Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0.
+
+Notice that the solution set must not contain duplicate triplets.
+
+### Constraints
+
+- 3 <= nums.length <= 3000
+- -10<sup>5</sup> <= nums[i] <= 10<sup>5</sup>
+
+### Example
+
+```
+Input: nums = [-1,0,1,2,-1,-4]
+Output: [[-1,-1,2],[-1,0,1]]
+Explanation:
+nums[0] + nums[1] + nums[2] = (-1) + 0 + 1 = 0.
+nums[1] + nums[2] + nums[4] = 0 + 1 + (-1) = 0.
+nums[0] + nums[3] + nums[4] = (-1) + 2 + (-1) = 0.
+The distinct triplets are [-1,0,1] and [-1,-1,2].
+Notice that the order of the output and the order of the triplets does not matter.
+```
+
+```
+Input: nums = [0,1,1]
+Output: []
+Explanation: The only possible triplet does not sum up to 0.
+```
+
+```
+Input: nums = [0,0,0]
+Output: [[0,0,0]]
+Explanation: The only possible triplet sums up to 0.
+```
+
+## Solution
+
+- No duplicates of triplets (Can I exclude with a hashmap?)
+- Brute force possible with an O(n<sup>3</sup>) solution
+- Approach like two sum and store pairs as key's and remainder as values?
+- All possibilities is n<sup>3</sup> (aka a cube) Some of these are complimentary, how can we eliminate those so we don't have to calc twice?
+
+### Pseudocode Attempt
+
+1. Create map
+2. Create result
+3. Iterate thru nums with i
+4. Iterate thru nums with j = i + 1
+5. Store 0 - nums[i] - nums[j] in map with [nums[i], nums[j], false]
+6. If found add to result and change to true, if true don't add to result
+7. return result
+
+### Initial Attempt
+
+```javascript
+const threeSum = function (nums) {
+  const map = {};
+  const result = [];
+
+  for (let i = 0; i < nums.length; i++) {
+    for (let j = i + 1; j < nums.length; j++) {
+      const remainder = 0 - nums[i] - nums[j];
+      if (remainder in map) {
+        // Check if combo was already used
+        if (!map[remainder][2]) {
+          result.push([remainder, nums[i], nums[j]]);
+        }
+      } else {
+        map[remainder] = [nums[i], nums[j], false];
+      }
+    }
+  }
+
+  return result;
+};
+```
+
+### Optimized Solution
+
+The time complexity of below is O(n<sup>2</sup>) with a space complexity likely O(n) due to the sorting. https://www.youtube.com/watch?v=jzZsG8n2R9A
+
+```javascript
+const threeSum = function (nums) {
+  const result = [];
+  nums.sort((a, b) => a - b);
+
+  for (let i = 0; i < nums.length; i++) {
+    // This is an optimization as it exits iterating over the sorted array when num[i] becomes positive. Only positive numbers would be left in the array and you can't add them to make zero
+    if (nums[i] > 0) {
+      break;
+    }
+
+    if (i > 0 && nums[i] === nums[i - 1]) {
+      continue;
+    }
+
+    let l = i + 1;
+    let r = nums.length - 1;
+    while (l < r) {
+      const total = nums[i] + nums[l] + nums[r];
+      if (total > 0) {
+        r--;
+      } else if (total < 0) {
+        l++;
+      } else {
+        result.push([nums[i], nums[l], nums[r]]);
+        l++;
+        while (nums[l] === nums[l - 1] && l < r) {
+          l++;
+        }
+      }
+    }
+  }
+
+  return result;
+};
+```