leetcode 221 Maximal Square

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

Credits:
Special thanks to @Freezen for adding this problem and creating all test cases.

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