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.

题解来自 http://www.geeksforgeeks.org/maximum-size-sub-matrix-with-all-1s-in-a-binary-matrix/
反正我觉得是好聪明，俺想不出来

1) Construct a sum matrix S[R][C] for the given M[R][C].
a)	Copy first row and first columns as it is from M[][] to S[][]
b)	For other entries, use following expressions to construct S[][]
If M[i][j] is 1 then
S[i][j] = min(S[i][j-1], S[i-1][j], S[i-1][j-1]) + 1
Else /*If M[i][j] is 0*/
S[i][j] = 0
2) Find the maximum entry in S[R][C]
3) Using the value and coordinates of maximum entry in S[i], print
sub-matrix of M[][]

public class Solution {
public int maximalSquare(char[][] matrix) {
if(matrix==null||matrix.length==0||matrix[0].length==0) return 0;
int m = matrix.length;
int n = matrix[0].length;
int[][] s = new int[m]
;

for(int i=0;i<m;i++){
if(matrix[i][0]=='1')
s[i][0]=1;
}
for(int j=1;j<n;j++){
if(matrix[0][j]=='1')
s[0][j]=1;
}
for(int i=1;i<m;i++){
for(int j=1;j<n;j++){
if(matrix[i][j]=='1'){
s[i][j] = Math.min(Math.min(s[i-1][j-1],s[i][j-1]),s[i-1][j])+1;
}else
s[i][j] = 0;
}
}
int res =s[0][0];
for(int i=0;i<m;i++){
for(int j=0;j<n;j++){
res = Math.max(res,s[i][j]);
}
}
return  res*res ;
}
}