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.

Classical DP problem.

int maximalSquare(vector<vector<char>>& matrix) {
    if(matrix.size() == 0) return 0;
    if(matrix[0].size() == 0) return 0;
    int m = matrix.size();
    int n = matrix[0].size();
    vector< vector<int> > dp(m, vector<int>(n, 0));
    for(int i = 0; i < m; ++i) {
        dp[i][0] = matrix[i][0] - '0';
    }
    for(int j = 0; j < n; ++j) {
        dp[0][j] = matrix[0][j] - '0';
    }

    for(int i = 1; i < m; ++i) {
        for(int j = 1; j < n; ++j) {
            if(matrix[i][j] == '0') dp[i][j] = 0;
            else {
                dp[i][j] = min(min(dp[i-1][j], dp[i][j-1]), dp[i-1][j-1]) + 1;
            }
        }
    }

    int maxSquare = 0;
    for(int i = 0; i < m; ++i) {
        for(int j = 0; j < n; ++j) {
            maxSquare = max(maxSquare, dp[i][j]);
        }
    }
    return maxSquare * maxSquare;   // here it asked for the square size.

    }