FZU Super A^B mod C（欧拉函数降幂）

#include <stdio.h>
#include <iostream>
#include <algorithm>
#include <cstdlib>
#include <sstream>
#include <numeric>
#include <cstring>
#include <bitset>
#include <string>
#include <deque>
#include <stack>
#include <cmath>
#include <queue>
#include <set>
#include <map>
using namespace std;
#define INF 0x3f3f3f3f
#define LC(x) (x<<1)
#define RC(x) ((x<<1)+1)
#define MID(x,y) ((x+y)>>1)
#define CLR(arr,val) memset(arr,val,sizeof(arr))
#define FAST_IO ios::sync_with_stdio(false);cin.tie(0);
typedef pair<int, int> pii;
typedef long long LL;
const double PI = acos(-1.0);
const int N = 1000010;

LL bpow(LL a, LL b, LL mod)
{
    LL r = 1LL;
    while (b)
    {
        if (b & 1)
            r = ((r % mod) * (a % mod)) % mod;
        a = ((a % mod) * (a % mod)) % mod;
        b >>= 1;
    }
    return r;
}
LL phi(LL n)
{
    LL r = n, sz = sqrt(n);
    for (LL i = 2; i <= sz; ++i)
    {
        if (n % i == 0)
        {
            r = r * (i - 1) / i;
            while (n % i == 0)
                n /= i;
        }
    }
    if (n > 1)
        r = r * (n - 1) / n;
    return r;
}
int main(void)
{
    LL a, c;
    char B[N];
    while (~scanf("%I64d%s%I64d", &a, B, &c))
    {
        LL phi_c = phi(c);
        int len = strlen(B);
        LL b;
        if (len <= 20)
        {
            sscanf(B, "%I64d", &b);
            if (b >= phi_c)
                b = b % phi_c + phi_c;
        }
        else
        {
            b = 0LL;
            for (int i = 0; i < len; ++i)
            {
                b = b * 10LL + B[i] - ‘0‘;
                if (b > phi_c)
                    b %= phi_c;
            }
            b += phi_c;
        }
        printf("%I64d\n", bpow(a, b, c));
    }
    return 0;
}