Acdream 1007 快速幂，模乘法

http://acdream.info/problem?pid=1007

 

题目大意，给你n个数，输出这n个数的k次方的和，注意最后的结果得是正数。

刚开始因为模乘法没有考虑好，一直死wa不过，后来在模乘法中特判一下，终于ac了。

 

#include<cstdio>
typedef long long LL;
const LL MOD = 10000000007LL;

int n;
LL k, temp, sum;

LL mul_mod(LL a, LL b)
{
    LL res = 0;
    int flag = (b < 0);
    b = (b ^ (b >> 63)) - (b >> 63);

    while(b)
    {
        if(b & 1)
            res = (res + a) % MOD;

        b >>= 1;
        a = (a << 1) % MOD;
    }

    res %= MOD;
    return (flag ? (~res + 1) : res);
}

LL calc(LL a, LL k)
{
    LL res = 1;

    while(k)
    {
        if(k & 1)
            res = mul_mod(a, res) % MOD;

        k >>= 1;
        a = mul_mod(a, a) % MOD;
    }

    return res;
}

int main()
{
    int tCase;
    scanf("%d", &tCase);

    while(tCase--)
    {
        sum = 0;
        scanf("%lld%lld", &n, &k);

        for(int i = 0; i < n; ++ i)
        {
            scanf("%lld", &temp);
            sum = (sum + calc(temp, k)) % MOD;
        }

        printf("%lld\n", (sum + MOD) % MOD);
    }

    return 0;
}