Solution.java

public class Solution {
    public int maximalSquare(char[][] matrix) {
        int cross=matrix.length;
        if (cross==0) {
            return 0;
        }
        int vertical=matrix[0].length;
        int max=0;
        int[][] size=new int[cross][vertical];
        for (int i = 0; i < cross; i++) {
            size[i][0]=matrix[i][0]=='1'?1:0;
            if (size[i][0]>max) {
                max=size[i][0];
            }
        }
        for (int i = 0; i < vertical; i++) {
            size[0][i]=matrix[0][i]=='1'?1:0;
            if (size[0][i]>max) {
                max=size[0][i];
            }
        }
        for (int i = 1; i < cross; i++) {
            for (int j = 1; j < vertical; j++) {
                size[i][j]=matrix[i][j]=='1'?Math.min(Math.min(size[i-1][j], size[i][j-1]), size[i-1][j-1])+1:0;
                if (size[i][j]>max) {
                    max=size[i][j];
                }
            }
        } 
        return max*max;
    }
}