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 01 0 1 1 11 1 1 1 11 0 0 1 0

Return 4.

dp[i][j] = min(dp[i-1][j-1], min(dp[i-1][j], dp[i][j-1])) + 1;//注意min

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

1 0 1 0 0 1 0 1 1 1 1 1 1 2 1 1 0 0 1 0

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