codeforces 810C Do you want a date? 数学 快速幂取模

题意：从一个含n个元素 的整数集中，取至少含有两个元素的子集，求所有子集a,,f（a）之和，输出结果为对1e9取模

思路：http://blog.csdn.net/dragon60066/article/details/72599167

#include<cstdio>#include<cstring>#include<algorithm>#define ll long longusing namespace std;const int maxn = 3* (1e5+10);const int mod = 1e9+7;int x[maxn];ll presum[maxn];ll ksm(ll a,ll b){ll base = a;ll ans=1;while(b){if(b&1)ans=(base*ans)%mod;base*=base;base%=mod;b=b>>1;}return ans;}int main(){int n;//freopen("in.txt","r",stdin);while(~scanf("%d",&n)){presum[0]=0;for(int i=1;i<=n;i++){scanf("%d",&x[i]);}sort(x+1,x+n+1);presum[0]=x[n]-x[1];for(int i=1;i<=n-2;i++)presum[i]=presum[i-1]+x[n-i]-x[i+1];ll ans=0;for(int i=0;i<=n-2;i++){//1 3 4ans = (ans+ksm(2,i)*(presum[i]%mod))%mod;}ans%=mod;printf("%I64d\n",ans);}}