update 200

Author: kaidi

Depth First Search/200. Number of Islands.py

@@ -0,0 +1,25 @@
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        if not grid:
+            return 0     
+        count = 0
+
+        for i in range(len(grid)):
+            for j in range(len(grid[0])):
+                if grid[i][j] == "1":
+                    self.dfs(grid, i, j)
+                    count +=1
+        return count
+        
+    def dfs(self,grid, i,j):
+        if i < 0 or j < 0 or i > len(grid)-1 or j > len(grid[0])-1 or grid[i][j]!="1":
+            return
+        grid[i][j] = '5'
+        self.dfs(grid,i-1,j)
+        self.dfs(grid,i+1,j)
+        self.dfs(grid,i,j-1)
+        self.dfs(grid,i,j+1)
+
+# Iterate all cells, for each cell, do a DFS
+# The exit situation of the DFS is when index is out of bound, or the current cell is not an island
+# Otherwise, if the current cell is still "1", change it to "5" (used as a flag, meaning this cell has been visited) 

Hashing/1512. Number of Good Pairs.py

@@ -0,0 +1,9 @@
+import math
+class Solution:
+    def numIdenticalPairs(self, nums: List[int]) -> int:
+        groups = collections.Counter(nums).values()
+        # result = sum([ n * (n-1)/2 for n in groups])
+        result = sum([math.comb(n,2) for n in groups])
+        return int(result)
+
+# Build in wheel for calculating the combinations.

README.md

@@ -14,13 +14,19 @@ For question details, please go to [Leetcode](https://leetcode.com/problemset/al
    + Medium 515 Find Largest Value in Each Tree Row --`[float('-inf')]`
 
 2. [Depth First Search]()
+   Iteration VS recursive.
+   ---
    + Pre-order traversal
    + In-order traversal
    + Post-order traversal
+   + Medium 200 Number of Islands 
 
 3. [Hashing](https://github.com/KaidiGuo/Algorithm-Exercises/tree/master/Hashing)
+   The use of collections.Counter() and dictionary.
+   ---
    + Easy 884 Uncommon Words from Two Sentences -- `[collections.Counter()]`
    + Easy 387 First Unique Character in a String -- `[collections.Counter()]`--`[float('inf')]`--`[index = string.find('c')]`
+   + Easy 1512. Number of Good Pairs `[math.comb(n,k)]`
    + Medium 347 Top K Frequent Elements -- `[counts = sorted(dic.values())]`
    + Easy 1 Two Sum -- `[for i,item in enumerate(nums)]`
    + Medium 3 Longest Substring Without Repeating Characters