hdu 3908 Triple

Triple

Time Limit: 5000/3000 MS (Java/Others)    Memory Limit: 125536/65536 K (Java/Others)
Total Submission(s): 205    Accepted Submission(s): 84

Problem Description

Given many different integers, find out the number of triples (a, b, c) which satisfy a, b, c are co-primed each other or are not co-primed each other. In a triple, (a, b, c) and (b, a, c) are considered as same triple.

 

Input

The first line contains a single integer T (T <= 15), indicating the number of test cases.
In each case, the first line contains one integer n (3 <= n <= 800), second line contains n different integers d (2 <= d < 10⁵) separated with space.

 

Output

For each test case, output an integer in one line, indicating the number of triples.

 

Sample Input

162 3 5 7 11 13

 

Sample Output

20

 

Source

2011 Multi-University Training Contest 7 - Host by ECNU

 

题解：这题比赛时卡了很久，后然才发现要用逆向思维，总数减去不满足的情况。

 

#include <iostream>using namespace std;int a[805];int n,ans;int gcd(int x,int y){if(y==0)return x;return gcd(y,x%y);}int main(){int i,j,T,x,y,s;scanf("%d",&T);while(T--){scanf("%d",&n);for(i=1;i<=n;i++)scanf("%d",&a[i]);ans=n*(n-1)*(n-2)/6;s=0;for(i=1;i<=n;i++){x=y=0;for(j=1;j<=n;j++){if(i==j)continue;if(gcd(a[i],a[j])==1)x++;elsey++;}s+=x*y;}printf("%d\n",ans-s/2);}return 0;}