BZOJ5091 摘苹果（概率期望）

　　大胆猜想每一步都相当于是第一步。稍微验证一下发现是对的。就做完了。

#include<iostream> 
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
#define ll long long
#define N 100010
#define P 1000000007
char getc(){char c=getchar();while ((c<'A'||c>'Z')&&(c<'a'||c>'z')&&(c<'0'||c>'9')) c=getchar();return c;}
int gcd(int n,int m){return m==0?n:gcd(m,n%m);}
int read()
{
    int x=0,f=1;char c=getchar();
    while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
    while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
    return x*f;
}
int n,m,k,a[N],degree[N],inv,ans;
int ksm(int a,int k)
{
    int s=1;
    for (;k;k>>=1,a=1ll*a*a%P) if (k&1) s=1ll*s*a%P;
    return s;
}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("bzoj5091.in","r",stdin);
    freopen("bzoj5091.out","w",stdout);
    const char LL[]="%I64d
";
#else
    const char LL[]="%lld
";
#endif
    n=read(),m=read(),k=read();
    for (int i=1;i<=n;i++) a[i]=read();
    for (int i=1;i<=m;i++)
    {
        int x=read(),y=read();
        degree[x]++,degree[y]++;
    }
    inv=ksm(2*m,P-2);
    for (int i=1;i<=n;i++) ans=(ans+1ll*a[i]*k%P*degree[i]%P*inv)%P;
    cout<<ans;
    return 0;
}