added Number of Islands (Medium)

Author: minhyeong-joe

Medium/NumberOfIslands/README.md

@@ -0,0 +1,38 @@
+# Number of Islands
+
+[Leetcode Link](https://leetcode.com/problems/number-of-islands/)
+
+## Problem:
+
+Given an `m x n` 2d grid map of `'1'`s (land) and `'0'`s (water), return the number of islands.
+
+An **island** is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.
+
+## Example:
+
+```
+Input: grid = [
+ ["1","1","1","1","0"],
+ ["1","1","0","1","0"],
+ ["1","1","0","0","0"],
+ ["0","0","0","0","0"]
+]
+Output: 1
+```
+
+```
+Input: grid = [
+ ["1","1","0","0","0"],
+ ["1","1","0","0","0"],
+ ["0","0","1","0","0"],
+ ["0","0","0","1","1"]
+]
+Output: 3
+```
+
+## Note:
+
+- m == grid.length
+- n == grid[i].length
+- 1 <= m, n <= 300
+- grid[i][j] is '0' or '1'.

Medium/NumberOfIslands/solution.py

@@ -0,0 +1,43 @@
+from typing import List
+
+
+class Solution:
+ def numIslands(self, grid: List[List[str]]) -> int:
+ count = 0
+ for i in range(0, len(grid)):
+ for j in range(0, len(grid[i])):
+ if grid[i][j] == "1":
+ count += 1
+ self.checkIslandDFS(grid, i, j)
+ return count
+
+ def checkIslandDFS(self, grid, i, j):
+ # base case
+ if i < 0 or j < 0 or i >= len(grid) or j >= len(grid[i]) or grid[i][j] == "0":
+ return
+
+ grid[i][j] = "0"
+ # check all adjacents
+ self.checkIslandDFS(grid, i, j-1) # left
+ self.checkIslandDFS(grid, i, j+1) # right
+ self.checkIslandDFS(grid, i-1, j) # top
+ self.checkIslandDFS(grid, i+1, j) # bottom
+
+
+sol = Solution()
+# grid = [
+# ["1", "1", "1", "1", "0"],
+# ["1", "1", "0", "1", "0"],
+# ["1", "1", "0", "0", "0"],
+# ["0", "0", "0", "0", "0"]
+# ]
+# print(sol.numIslands(grid))
+
+# grid = [
+# ["1", "1", "0", "0", "0"],
+# ["1", "1", "0", "0", "0"],
+# ["0", "0", "1", "0", "0"],
+# ["0", "0", "0", "1", "1"]
+# ]
+grid = [["1", "1", "1"], ["0", "1", "0"], ["1", "1", "1"]]
+print(sol.numIslands(grid))

README.md

@@ -85,6 +85,7 @@ Languages used: Java and Python
  - [Score After Flipping Matrix](Medium/ScoreAfterFlippingMatrix)
  - [Construct Binary Tree from Preorder and Inorder Traversal](Medium/ConstructBinaryTree)
  - [K Closest Points to Origin](Medium/KClosestPointsToOrigin)
+ - [Number of Islands](Medium/NumberOfIslands)
 - Hard
  - [Maximum Score Words Formed by Letters](Hard/MaximumScoreWords)
  - [Reducing Dishes](Hard/ReducingDishes)