看了斌神的题解恍然大悟
代码：
#include <stdio.h>#include <string.h>#include <algorithm>using namespace std;typedef long long ll;const int MOD = 1000000007;const int N = 200005;int t, n, m, f[N], rf[N];int pow_mod(int x, int k) {int ans = 1;while (k) {if (k&1) ans = (ll)ans * x % MOD;x = (ll)x * x % MOD;k >>= 1;}return ans;}int C(int n, int m) {return (ll)f[n] * rf[n – m] % MOD * rf[m] % MOD;}int cal(int x, int n, int sum) {if (n == 0) return sum == 0;int ans = 0;for (int i = 0; i * x <= sum && i <= n; i++) {int tmp = (ll)C(n, i) * C(sum – i * x + n – 1, n – 1) % MOD;if (i%2) ans = ((ans – tmp) % MOD + MOD) % MOD;else ans = (ans + tmp) % MOD;}return ans;}int main() {f[0] = 1;for (int i = 1; i < N; i++)f[i] = (ll)f[i – 1] * i % MOD;rf[N – 1] = pow_mod(f[N – 1], MOD – 2);for (int i = N – 2; i >= 0; i–)rf[i] = (ll)rf[i + 1] * (i + 1) % MOD;scanf("%d", &t);while (t–) {scanf("%d%d", &n, &m);int ans = 0;for (int i = 1; i <= n; i++)ans = (ans + cal(i, m – 1, n – i)) % MOD;printf("%d\n", ans);}return 0;}
，要知道，当你一直在担心错过了什么的时候，其实你已经错过了旅行的意义。
