221. Maximal Square

class Solution {
public:
    int maximalSquare(vector<vector<char>>& matrix) {
        int m = matrix.size(); if (m == 0)  return 0;
        int n = matrix[0].size(); if (n == 0) return 0;
        vector<vector<int>> dp(m+1, vector<int>(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;
    }
};