等比数列求和 二分思想

AC代码

 1 #include <stdio.h>
 2 #include <math.h>
 3 #include <string.h>
 4 #include <stdlib.h>
 5 #include <iostream>
 6 #include <sstream>
 7 #include <algorithm>
 8 #include <string>
 9 #include <queue>
10 #include <map>
11 #include <vector>
12 using namespace std;
13 const int maxn = 1e6+50;
14 const int inf = 0x3f3f3f3f,mod = 1000000007;
15 const double epx = 1e-10;
16 typedef long long ll;
17 ll poww(ll n,ll m)
18 {
19     ll ans = 1;
20     while(m > 0)
21     {
22         if(m & 1)ans = (ans * n) % mod;
23         m = m >> 1;
24         n = (n * n) % mod;
25     }
26     return ans;
27 }
28 ll sum(ll a,ll n)
29 {
30     if(n==1) return a;
31     ll t=sum(a,n/2);
32     if(n&1)
33     {
34         ll cur=poww(a,n/2+1)%mod;
35         t=(t+(t*cur)%mod)%mod;
36         t=(t+cur)%mod;
37     }
38     else
39     {
40         ll cur=poww(a,n/2)%mod;
41         t=(t+(t*cur)%mod)%mod;
42     }
43 }
44 int main()
45 {
46     ll a,n;
47     cin>>a>>n;
48     cout<<sum(a,n)<<endl;
49 }

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