221. Maximal Square

Given a 2D binary matrix filled with 0's and 1's, find the largest square containing only 1's and return its area.

Example:

Input: 

1 0 1 0 0
1 0 1 1 1
1 1 1 1 1
1 0 0 1 0

Output: 4

用dp。二重循环，如果dp[i][j]=1，检查一下它的上、左、右，取三个值中的最小值+1 （dp[i][j]），再更新一下max。

注意dp的长度比matrix多1.

时间：O(N^2)，空间O(N^2)

class Solution {
    public int maximalSquare(char[][] matrix) {
        if(matrix == null || matrix.length == 0 || matrix[0].length == 0) return 0;
        
        int m = matrix.length, n = matrix[0].length, max = 0;
        int[][] dp = new int[m+1][n+1];
        
        for(int i = 1; i <= m; i++) {
            for(int j = 1; j <= n; j++) {
                if(matrix[i-1][j-1] == '1') {
                    dp[i][j] = Math.min(dp[i-1][j-1], Math.min(dp[i][j-1], dp[i-1][j])) + 1;
                    max = Math.max(max, dp[i][j]);
                }
            }
        }
        return max * max;
    }
}