https://www.cnblogs.com/grandyang/p/4550604.html 解法3

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