Maximal Square

public class Solution {
    public int maximalSquare(char[][] matrix) {
        if(matrix==null || matrix.length==0 || matrix[0].length==0) return 0;
        int row = matrix.length;
        int col = matrix[0].length;
        int max = 0;
        int[][] square = new int[row][col];
        //first row
        for(int i=0; i<row; i++)
            square[i][0] = matrix[i][0]-‘0‘;
        //first col
        for(int j=0; j<col; j++)
            square[0][j] = matrix[0][j]-‘0‘;
        for(int i=1; i<row; i++)
        {
            for(int j=1; j<col; j++)
            {
                if(matrix[i][j] == ‘0‘)     square[i][j] = 0;
                else    square[i][j] = Math.min(Math.min(square[i-1][j], square[i][j-1]), square[i-1][j-1])+1;
            }
        }
        for(int i=0; i<row; i++)
        {
            for(int j=0; j<col; j++)
            {
                max = Math.max(max, square[i][j]);
            }
        }
        
        return max*max;
    }
}