【Luogu P3799】Demon Dream Fighting Wooden Stick 【Combination Mathematics】

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Problem solving ideas

To put it bluntly, choose two equal sticks, and then pick the two that add up to the original two sticks, and combine them.

Because $m$ is equal to2 2in this question$2$ , then we can simplify it to get:$2/n(n-1)$

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Code

#include<algorithm>
#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
using namespace std;
const long long INF=1000000007;
long long n,a[100010],maxn,ans,x;

int main(){
    
    
	scanf("%lld",&n);
	for(int i=1;i<=n;i++)
	{
    
    
		scanf("%lld",&x);
		a[x]++;
		maxn=max(maxn,x);
	}	
	for(int i=2;i<=maxn;i++)
	{
    
    
		if(a[i]>1)
		{
    
    
			for(int j=1;j<=i/2;j++)
			{
    
    
				if((i-j)!=j&&a[j]&&a[i-j])
					ans=(ans+(((a[i]*(a[i]-1)/2)*a[j]*a[i-j]))%INF)%INF;
				if((i-j)==j&&a[j]>1)
					ans=(ans+(((a[i]*(a[i]-1)/2)*(a[j]*(a[j]-1)/2)))%INF)%INF;
			}
		}
	}
	printf("%lld",ans);
}