leetcode 221 maximal square

class  {
    public int maximalSquare(char[][] matrix) {
        if(matrix.length == 0) return 0;
        if(matrix[0].length == 0) return 0;
        int m = matrix.length;
        int n = matrix[0].length;
        
        int[][] dp = new int[m+1][n+1];
        int size = 0;
        for(int i=1; i<=m; i++){
            for(int j=1; j<=n; j++){
                if(matrix[i-1][j-1] == '1'){
                    dp[i][j] = Math.min(dp[i-1][j-1], Math.min(dp[i-1][j], dp[i][j-1])) + 1;
                    size = Math.max(size, dp[i][j]);
                }
            }
        }
        return size*size;
    }
}