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

For example, given the following matrix:

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

Return 4.

tag: DP

class Solution {
    public int maximalSquare(char[][] matrix) {
        if (matrix == null || matrix.length == 0) return 0;
        int n = matrix.length, m = matrix[0].length;

        int[][] dp = new int[n][m];
        int ans = 0;
        for (int i = 0; i < n; i++){
            for (int j = 0; j < m; j++){
                if (i == 0 || j == 0){
                    dp[i][j] = matrix[i][j] - '0';
                }
                else{
                    dp[i][j] = matrix[i][j] == '1' ? Math.min(dp[i - 1][j - 1], Math.min(dp[i - 1][j], dp[i][j - 1])) + 1 : 0;
                }
                ans = Math.max(ans, dp[i][j]);
            }
        }

        return ans * ans;
    }
}