Nastya and a Wardrobe CodeForces - 992C（规律）

写一下二叉树  推一下公式就出来了， 注意取模时的输出形式

#include <bits/stdc++.h>
#define mem(a, b) memset(a, b, sizeof(a))
using namespace std;
typedef long long LL;
const int maxn = 1e6+5, INF = 0x7fffffff, MOD = 1e9+7;
LL n, k;

LL qp(LL a, LL b)
{
    LL res = 1;
    while(b)
    {
        if(b & 1) res = res * a % MOD;
        a = a * a % MOD;
        b >>= 1;
    }
    return res;
}

int main()
{
    while(cin>> n >> k)
    {
        if(n == 0)
        {
            cout<< 0 <<endl;
            return 0;
        }
        n %= MOD;
        cout<< ((qp(2, k+1) * n % MOD - (qp(2, k) - 1 + MOD) % MOD) + MOD) % MOD <<endl;
    }

    return 0;
}

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