hdu 4602 Partition（计数）

题目链接：hdu 4602 Partition

代码

#include <cstdio>#include <cstring>#include <algorithm>using namespace std;const int mod = 1e9 + 7;int N, K;long long pow_mod(long long x, int n) {    if (n < 0) return 0;    long long ret = 1;    while (n) {        if (n&1) ret = ret * x % mod;        x = x * x % mod;        n >>= 1;    }    return ret;}int main () {    int cas;    scanf("%d", &cas);    while (cas--) {        scanf("%d%d", &N, &K);        if (N == K) {            printf("1\n");            continue;        }        int ans = pow_mod(2, N-K);        ans = (ans + pow_mod(2, N-K-2) * (N-K-1)) % mod;        printf("%d\n", ans);    }    return 0;}