class Solution {public:    int maximalSquare(vector
     
      
       >& matrix) {        int m = matrix.size(); if (m == 0)  return 0;        int n = matrix[0].size(); if (n == 0) return 0;        vector
       
        
         > dp(m+1, vector
         
          (n+1,0));        int res = 0;        for (int i = 1; i <= m; i++)            for (int j = 1; j <= n; j++) {                if (matrix[i-1][j-1] == '0')                    dp[i][j] = 0;                else {                    dp[i][j] = min(dp[i-1][j], min(dp[i][j-1], dp[i-1][j-1])) + 1;                    res = max(res, dp[i][j]);                }            }        return res * res;    }};