【算法】求岛的数量（***Number of Islands**

给定一个2D的格子Map,1 表示陆地 ，0 表示水，计算岛的数量
岛是被水环绕和相邻的陆地水平方向或者垂直方向相连，可以认为岛的边界都是水包围

例子1;

Input:
11110
11010
11000
00000
Output: 1

例子2：

Input:
11000
11000
00100
00011
Output: 3

class Solution {
   private int n;
private int m;

public int numIslands(char[][] grid) {
    int count = 0;
    n = grid.length;
    if (n == 0) return 0;
    m = grid[0].length;
    for (int i = 0; i < n; i++){
        for (int j = 0; j < m; j++)
            if (grid[i][j] == '1') {
                DFSMarking(grid, i, j);
                ++count;
            }
    }    
    return count;
}

private void DFSMarking(char[][] grid, int i, int j) {
    if (i < 0 || j < 0 || i >= n || j >= m || grid[i][j] != '1') return;
    grid[i][j] = '0';
    DFSMarking(grid, i + 1, j);
    DFSMarking(grid, i - 1, j);
    DFSMarking(grid, i, j + 1);
    DFSMarking(grid, i, j - 1);
}
}