HDU 2685 I won't tell you this is about number theory

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=2685

题意：求gcd(a^m - 1, a^n - 1) mod k

思路：gcd(a^m - 1, a^n - 1) = a^gcd(m, n) - 1

code：

 #include <stdio.h>

 int gcd(int a, int b)
 {
     return !b ? a : gcd(b, a%b);
 }

 int mod_pow(int a, int x, int mod)
 {
     int tmp = a;
     int ret = ;
     while(x) {
         if (x & ) {
             ret = ret * tmp % mod;
         }
         tmp = tmp * tmp % mod;
         x >>= ;
     }

     return ret;
 }

 int main()
 {
     int t, a, m, n, k;
     scanf("%d", &t);
     while (t--) {
         scanf("%d %d %d %d", &a, &m, &n, &k);
         int d = gcd(m, n);
         int ans = mod_pow(a, d, k);
         printf("%d\n", (ans -  + k) % k);
     }

     return ;
 }