1. Leetcode 200 岛屿数量

leetcode.cn/problems/nu…

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

        for (int i = 0; i < grid.length; i++) {
            for (int j = 0; j < grid[0].length; j++) {
                // 如果找到了岛屿，则将其标记更新为2，并将与其上下左右链接的岛屿更新为2
                if (grid[i][j] == '1') {
                    res++;
                    dfs(grid, i, j);
                }
            }
        }

        return res;
    }

    /**
     * 如果grid[i][j]为岛屿，则标记所有与其相邻的岛屿; 如果grid[i][j]不是岛屿或者(i,j)越界，则停止遍历
     * @param grid
     * @param i
     * @param j
     */
    private void dfs(char[][] grid, int i, int j) {
        // DFS过程中如果超过边界或者当前遍历到的元素为海洋，则停止遍历
        if (i < 0 || i >= grid.length || j < 0 || j >= grid[0].length || grid[i][j] != '1') {
            return;
        }


        if (grid[i][j] == '1') {
            grid[i][j] = '2';
        }

        dfs(grid, i - 1, j);
        dfs(grid, i + 1, j);
        dfs(grid, i, j - 1);
        dfs(grid, i, j + 1);
    }
}