LeetCode 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:

11110
11010
11000
00000

Answer: 1

Example 2:

11000
11000
00100
00011

Answer: 3

可惜这样的题估计不会出现在实际中

class Solution {
public:
    int numIslands(vector<vector<char>> &grid) {
        int ylen = grid.size();
        if (ylen == 0) return 0;
        int xlen = grid[0].size();
        if (xlen == 0) return 0;
        
        int island = 0;
        
        for (int y = 0; y<ylen; y++) {
            for (int x = 0; x<xlen; x++) {
                if (dfs(grid, x, y)) island++;
            }
        }
        return island;
    }
    
    bool dfs(vector<vector<char>> &grid, int x, int y) {
        int ylen = grid.size();
        if (ylen == 0) return false;
        
        int xlen = grid[0].size();
        if (xlen == 0) return false;
        
        if (x < 0 || x >= xlen) return false;
        if (y < 0 || y >= ylen) return false;
        
        if (grid[y][x] == '0') return false;
        
        grid[y][x] = '0'; // visit it
        
        dfs(grid, x + 1, y);
        dfs(grid, x - 1, y);
        dfs(grid, x, y + 1);
        dfs(grid, x, y - 1);
        return true;
    }
};