HDU 5201 The Monkey King(容斥)

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看了斌神的题解恍然大悟http://www.kuangbin.net/archives/bc36

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;}

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