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

解题思路

深度优先搜索，要求的结果就是图中连通分量的个数。更详细的思路可以参考Find the number of islands。

实现代码

//Runtime:13ms
class Solution {
public:
    int numIslands(vector<vector<char>> &grid) {
        if (grid.empty() || grid[0].empty()) return 0;
        int row = grid.size();
        int col = grid[0].size();
        int cnt = 0;
        for (int i = 0; i < row; i++)
        {
            for (int j = 0; j < col; j++)
            {
                if (grid[i][j] == ‘1‘)
                {
                    cnt++;
                    dfs(grid, i, j);
                }
            }
        }

        return cnt;
    }

private:
    void dfs(vector<vector<char>> &grid, int row, int col)
    {
        if (row < 0 || row >= grid.size() || col < 0 || col >= grid[0].size() || grid[row][col] != ‘1‘)
        {
            return;
        }

        grid[row][col] = ‘X‘;
        dfs(grid, row, col + 1);
        dfs(grid, row + 1, col);
        dfs(grid, row, col - 1);
        dfs(grid, row - 1, col);
    }
};