[Leetcode 695]岛屿最大面积 Max Area of Island DFS

 

 

【题目】

岛屿矩阵，0表示海洋，数字表示岛屿面积，只有上下左右代表岛屿相连，对角线不算。

求问海洋里最大的岛屿面积

You are given an m x n binary matrix grid. An island is a group of 1's (representing land) connected 4-directionally (horizontal or vertical.) You may assume all four edges of the grid are surrounded by water.

The area of an island is the number of cells with a value 1 in the island.

Return the maximum area of an island in grid. If there is no island, return 0.

 

Example 1:

Input: grid = [[0,0,1,0,0,0,0,1,0,0,0,0,0],[0,0,0,0,0,0,0,1,1,1,0,0,0],[0,1,1,0,1,0,0,0,0,0,0,0,0],[0,1,0,0,1,1,0,0,1,0,1,0,0],[0,1,0,0,1,1,0,0,1,1,1,0,0],[0,0,0,0,0,0,0,0,0,0,1,0,0],[0,0,0,0,0,0,0,1,1,1,0,0,0],[0,0,0,0,0,0,0,1,1,0,0,0,0]]
Output: 6 最大面积6
Explanation: The answer is not 11, because the island must be connected 4-directionally.

Example 2:

Input: grid = [[0,0,0,0,0,0,0,0]]
Output: 0

 

【思路】

用DFS，遍历上下左右是否相连四种情况。

注意边界情况，grid的length。

当遇到海洋时，结束。因此对访问计算过的岛屿标记成海洋。

 

【代码】

 

class Solution {
public int maxAreaOfIsland(int[][] 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]!=0){
                    ans=Math.max(dfs(grid,i,j,0),ans);}}}
        return ans;

    }

    public int dfs(int[][] grid,int i,int j,int ans){
        if (i < 0 || i >= grid.length || j < 0 || j >= grid[0].length || grid[i][j] == 0) {
            return ans;
        }
        
        ans+=grid[i][j];
        grid[i][j]=0;
        //已经遍历过的岛屿标记为0海洋

        ans=dfs(grid,i+1,j,ans);
        ans=dfs(grid,i-1,j,ans);
        ans=dfs(grid,i,j+1,ans);
        ans=dfs(grid,i,j-1,ans);
        //上下左右，ans调用累加
        return ans;
    }
}