codeforces 822D
2017-07-04 20:31
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题目链接:
http://codeforces.com/problemset/problem/822/D题解:
预处理很重要。代码:
#include <cstdio> #include <cmath> #include <cstring> #include <iostream> #include <algorithm> using namespace std; #define met(a,b) memset(a,b,sizeof(a)) #define inf 0x3f3f3f3f const int maxn = 5*1e6; typedef long long ll; const int mod = 1e9+7; ll num[maxn+10]; ll prime[maxn+10]; void init() { met(num,0); met(prime,0); for(ll i=2;i<=maxn;i++) { if(num[i]==0) { for(ll j=i*i;j<=maxn;j+=i) { if(num[j]==0) num[j]=i; } } } for(ll i=2;i<=maxn;i++) { if(num[i]==0) prime[i]=(i*(i-1)/2)%mod; else { prime[i]=((prime[num[i]]*(i/num[i])%mod)%mod+prime[i/num[i]])%mod; } } } int main() { init(); int t,l,r; scanf("%d%d%d",&t,&l,&r); ll sum=0,tt=1; for(ll i=l;i<=r;i++) {sum=(sum+(tt*prime[i])%mod)%mod;tt=(tt*t)%mod;} cout<<sum<<endl; }
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