【Medium】200. Number of Islands

Given a 2d grid map of '1's (land) and '0's (water), count 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 1:

11110110101100000000

Answer: 1

Example 2:

11000110000010000011

Answer: 3

中心思想：

DFS+Recursion

如果一个格子是1，将其标为0后，查其上下左右，重复把查到的1标为0。当一个岛全为0后，recursion会停止，记住找到了一个岛，然后继续找地图中的下一个1.

class Solution { public: int numIslands(vector<vector<char>>& grid) { int n = grid.size(); if (n == 0){ return 0; } int m = grid[0].size(); int counter = 0; for (int i = 0; i < n; i++){ for (int j = 0; j < m; j++){ if (grid[i][j] == '1'){ grid[i][j] = '0'; counter++; search(i, j, grid); } } } return counter; } void search(int i, int j, vector<vector<char>>& grid){ int n = grid.size(); int m = grid[0].size(); if (grid[i][j] == '1') grid[i][j] = '0'; if (i > 0 && grid[i-1][j] == '1') search(i-1, j, grid); if (i < n-1 && grid[i+1][j] == '1') search(i+1, j, grid); if (j > 0 && grid[i][j-1] == '1') search(i, j-1, grid); if (j < m-1 && grid[i][j+1] == '1') search(i, j+1, grid); } };