Leetcode Hot100 81-85

dfs框架代码

void dfs(int[][] grid, int r, int c) {
    // 判断 base case
    if (!inArea(grid, r, c)) {
        return;
    }
    // 如果这个格子不是岛屿，直接返回
    if (grid[r][c] != 1) {
        return;
    }
    grid[r][c] = 2; // 将格子标记为「已遍历过」
    
    // 访问上、下、左、右四个相邻结点
    dfs(grid, r - 1, c);
    dfs(grid, r + 1, c);
    dfs(grid, r, c - 1);
    dfs(grid, r, c + 1);
}

// 判断坐标 (r, c) 是否在网格中
boolean inArea(int[][] grid, int r, int c) {
    return 0 <= r && r < grid.length 
        	&& 0 <= c && c < grid[0].length;
}

思路：遇到陆地就ans+1，然后把周围的陆地都变为海洋 

class Solution {
    public int numIslands(char[][] grid) {
        int ans = 0;
        for (int i = 0; i < grid.length; i++) {
            for (int j = 0; j < grid[0].length; j++) {
                if(grid[i][j]=='1'){
                    dfs(grid,i,j);
                    ans++;
                }
            }
        }
        return ans;
    }
    public void dfs(char[][]grid,int i,int j){
        if(i<0||i>grid.length-1||j<0||j>grid[0].length-1||grid[i][j]=='0'){
            return;
        }
        grid[i][j] = '0';
        dfs(grid, i+1, j);
        dfs(grid, i-1, j);
        dfs(grid, i, j+1);
        dfs(grid, i, j-1);
    }
}