2013
12-11

# Homogeneous squares

Assume you have a square of size n that is divided into n×n positions just as a checkerboard. Two positions (x1,y1) and (x2,y2), where 1 ≤ x1,y1,x2,y2 ≤ n, are called “independent” if they occupy different rows and different columns, that is, x1≠x2 and y1≠y2. More generally, n positions are called independent if they are pairwise independent. It follows that there are n! different ways to choose n independent positions.

Assume further that a number is written in each position of such an n×n square. This square is called “homogeneous” if the sum of the numbers written in n independent positions is the same, no matter how the positions are chosen. Write a program to determine if a given square is homogeneous!

The input contains several test cases.
The first line of each test case contains an integer n (1 ≤ n ≤ 1000). Each of the next n lines contains n numbers, separated by exactly one space character. Each number is an integer from the interval [-1000000,1000000].
The last test case is followed by a zero.

For each test case output whether the specified square is homogeneous or not. Adhere to the format shown in the sample output.

2
1 2
3 4
3
1 3 4
8 6 -2
-3 4 0
0

homogeneous
not homogeneous

Idea：
1. 搜索？N！的空间（N＝１０００），不行
2. 数学规律了（我直接去搜题解了），下面是ｃｏｐｙ的：

<== 对于n*n（n>=2）的矩阵，如果所有的2*2子矩阵都为“Homogeneous ”，那么可以得到a[i-1][j]- a[i-1][j-1] == a[i][j] – a[i][j-1] == kj（n > j > 0, n > i > 0）,也就是说这个差是与i无关，那么，原始矩阵a可以写成两个矩阵b,c的和。b[i][0]= a[i][0](n > i >= 0)，其他元素为0; c[i][j] = sum(kj),(n > j > 0).这样c的每一行元素都一样，每次取a的一个序列（index_0,index_1…index_(n-1)）时，它们的和必然相同，因为这个和衡等于sum(b[i][0])＋sum(ki) （n > i >= 0）。
==>

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#include <cstdio>
#include <iostream>
#include <fstream>
#include <cstring>
#include <string>
#define OP(s) cout<<#s<<"="<<s<<" ";
#define PP(s) cout<<#s<<"="<<s<<endl;
using namespace std;

int main()
{
freopen("test.txt","r",stdin);
static int a[1010],b[1010];
static char s[3010];
int N;
while (scanf("%d",&N),N)
{
a[0] = 0;
for (int i = 1; i <= N; i++)
{
scanf("%d",&a[i]);
b[i] = a[i] - a[i-1];
}

bool ans = true;
for (int i = 2; i <= N; i++)
{
if (ans)
for (int j = 1; j <= N; j++)
{
scanf("%d",&a[j]);
if (j != 1)
{
if (a[j] - a[j-1] != b[j]) ans = false;
}
}
else {getchar();gets(s);}
}

if (ans) printf("homogeneous\n");
else printf("not homogeneous\n");
}

return 0;
}