【HDOJ】4704 Sum

数学题。f(n) = 2^(n-1) mod (1e9+7)。

#include <cstdio>

#define MAXN 100005

char buf[MAXN];
__int64 phi = 1e9+6;
__int64 mod = 1e9+7;

__int64 power2(__int64 n) {
    __int64 ret = 1, base = 2;

    --n;
    while (n) {
        if (n & 1)
            ret = ret * base % mod;
        base = base*base%mod;
        n >>= 1;
    }
    return ret;
}

int main() {
    __int64 tmp;
    __int64 i, j;

    while (scanf("%s", buf) != EOF) {
        tmp = 0;
        for (i=0; buf[i]; ++i) {
            tmp = 10*tmp + (buf[i]-'0');
            tmp %= phi;
        }
        tmp += phi;
        tmp = power2(tmp);
        printf("%I64d
", tmp);
    }

    return 0;
}