HDU 4602

EASY PROBLEM

#include <cstdio>
#include <iostream>
#include <cstring>
using namespace std;
typedef long long LL;
const int maxn = 111111;
const int mod = (int) (1e9 + 7);
char s[maxn];
int num[maxn];
LL POW(LL a, LL b) {
    LL ans = 1, t = a;
    while (b) {
        if (b & 1)
            ans = ans * t % mod;
        t = t * t % mod;
        b >>= 1;
    }
    return ans;
}
LL solve(LL n, LL k) {
    if (k > n)
        return 0;
    if (k == n)
        return 1;
    if (k == n - 1)
        return 2;
    LL a = POW(2, n - k - 2), b = POW(2, n - k);
    LL ans = a * (n - k - 1) + b;
    ans = (ans % mod + mod) % mod;
    return ans;
}
int main() {
    ios::sync_with_stdio(false);
    int T;
    cin >> T;
    while (T--) {
        LL n, k;
        cin >> n >> k;
        cout << solve(n, k) << endl;
    }
    return 0;
}