岛屿个数

题目：求岛屿数量
dfs标记位grid[r][[c] = 0; 告诉已经走过，是否需要重置标志位，需要根据dfs是否需要走回头路来判断。

void dfs(char[][] grid, int r, int c) {
    
    
        int nr = grid.length;
        int nc = grid[0].length;

        if (r < 0 || c < 0 || r >= nr || c >= nc || grid[r][c] == '0') {
    
    
            return;
        }

        grid[r][c] = '0';
        dfs(grid, r - 1, c);
        dfs(grid, r + 1, c);
        dfs(grid, r, c - 1);
        dfs(grid, r, c + 1);
    }

    public int numIslands(char[][] grid) {
    
    
        if (grid == null || grid.length == 0) {
    
    
            return 0;
        }

        int nr = grid.length;
        int nc = grid[0].length;
        int num_islands = 0;
        for (int r = 0; r < nr; ++r) {
    
    
            for (int c = 0; c < nc; ++c) {
    
    
                if (grid[r][c] == '1') {
    
    
                    ++num_islands;
                    dfs(grid, r, c);
                }
            }
        }

        return num_islands;
    }