hdu 5419(数学期望)

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;

typedef long long LL;
const int maxn = 50005;
int n,m,w[maxn];
int bound[maxn];

LL gcd(LL a,LL b)
{
	if(b) return gcd(b,a % b);
    return a;
}

LL C(LL a)
{
	return a * (a - 1) * (a - 2) / 6;
}

int main()
{
	int t,l,r;
	scanf("%d",&t);
	while(t--)
	{
		memset(bound,0,sizeof(bound));
		scanf("%d%d",&n,&m);
		for(int i = 1; i <= n; i++)
			scanf("%d",&w[i]);
		for(int i = 1; i <= m; i++)
		{
			scanf("%d%d",&l,&r);
			bound[l]++; bound[r+1]--;
		}
		int cnt = 0;
		LL ans1 = 0;
		for(int i = 1; i <= n; i++)
		{
			cnt += bound[i];
			if(cnt >= 3)
				ans1 += w[i] * C(cnt);
		}
		LL ans2 = C(m);
		LL d = gcd(ans1,ans2);
		if(d > 0)
		{
			ans1 /= d; ans2 /= d;
			if(ans2 == 1) printf("%lld\n",ans1);
			else printf("%lld/%lld\n",ans1,ans2);
		}
		else printf("0\n");
	}
	return 0;
}