【Luogu_P3799】Demon dream fight wooden stick

Demon dream fight wooden stick

Thank you @Jackma_mayichao for your help

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Topic link: Monster Dream Fighting Wooden Stick

Problem solving ideas

Perceptually understand the meaning of the question: use 4 wooden sticks to form an equilateral triangle . To put it bluntly , choose two equal sticks, and then pick two sticks that add up to the original two sticks, and combine them.

Then we look at combinatorics :

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

code

#include<iostream>
#include<cstdio>
#define int long long
using namespace std;

const int mod=1e9+7;

int n,a[5010];
int maxn,ans;

signed main()
{
    
    
	cin>>n;
	for(int i=1;i<=n;i++)
	{
    
    
		int t;
		scanf("%lld",&t);
		a[t]++;
		maxn=max(maxn,t);
	}
	for(int i=2;i<=maxn;i++)
	{
    
    
		if(a[i]<=1)
			continue;
		for(int j=1;j<=i/2;j++)
		{
    
    
			if(i-j!=j&&a[i]&&a[j])
				ans=(ans+(a[i]*(a[i]-1)/2*a[j]*a[i-j])%mod)%mod;
			else if(i-j==j&&a[j]>=1)
				ans=(ans+((a[i]*(a[i]-1)/2)%mod*(a[j]*(a[j]-1)/2)%mod)%mod)%mod;
		}
	}
	cout<<ans<<endl; 
}