牛客網小a的旅行計劃
阿新 • • 發佈:2018-11-03
地址:https://ac.nowcoder.com/acm/contest/223/B
此題是一道套公式題目,不難,ans=(pow(4,n+1)-4*pow(3,n+1)+6*pow(2,n+1)-4)*pow(8,mod-2)
#include <cstdio> #include <queue> #include <cstring> #include <iostream> #include <cstdlib> #include <algorithm> #include <vector> #include<map> #include <cmath> using namespace std; typedef long long ll; const ll mod=1e8+7; ll qow(ll a,ll b) { if(b==0)return 1; if(b==1)return a%mod; ll sum=qow(a,b/2); return b%2?(a*((sum*sum)%mod))%mod:(sum*sum)%mod; } int main() { ll n; cin>>n; n++; ll ans=((((qow(4,n)-4*qow(3,n)+6*qow(2,n)-4)%mod+mod)%mod)*qow(8,mod-2))%mod; cout<<ans<<endl; return 0; }