等比数列求和 二分思想

AC代码

#include <stdio.h>
#include <math.h>
#include <string.h>
#include <stdlib.h>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <string>
#include <queue>
#include <map>
#include <vector>
using namespace std;
const int maxn = 1e6+50;
const int inf = 0x3f3f3f3f,mod = 1000000007;
const double epx = 1e-10;
typedef long long ll;
ll poww(ll n,ll m)
{
    ll ans = 1;
    while(m > 0)
    {
        if(m & 1)ans = (ans * n) % mod;
        m = m >> 1;
        n = (n * n) % mod;
    }
    return ans;
}
ll sum(ll a,ll n)
{
    if(n==1) return a;
    ll t=sum(a,n/2);
    if(n&1)
    {
        ll cur=poww(a,n/2+1)%mod;
        t=(t+(t*cur)%mod)%mod;
        t=(t+cur)%mod;
    }
    else
    {
        ll cur=poww(a,n/2)%mod;
        t=(t+(t*cur)%mod)%mod;
    }
    return t;
}
int main()
{
    ll a,n;
    cin>>a>>n;
    cout<<sum(a,n)<<endl;
}

 参考博客 https://blog.csdn.net/acdreamers/article/details/7851144