逆元模板

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<cmath>
 5 using namespace std;
 6 int a,b,m;
 7 int x,y;
 8 int exgcd(int a,int b,int &x,int &y)
 9 {
10     if(b==0)
11     {
12         x=1;
13         y=0;
14         return a;
15     }
16     int r=exgcd(b,a%b,x,y);
17     int tmp=x;
18     x=y;
19     y=tmp-a/b*y;
20     return r;
21 }
22 int fastpow(int a,int p)
23 {
24     int base=a;int ans=1;
25     while(p!=0)
26     {
27         if(p%2==1)ans=ans*base;
28         base=base*base;
29         p=p/2;
30     }
31     return ans;
32 }
33 int main()
34 {
35     scanf("%d%d%d",&a,&b,&m);
36     for(int i=1;i<=sqrt(m);i++)
37     {
38         if(m%i==0)
39         {
40             int ans=exgcd(b,m,x,y);
41             printf("%d",(a*ans)%m);
42             return 0;    
43         }
44     }
45     printf("%d",fastpow(b,m-2));
46 }