HDU 2068（错排+组合数）

本体要用long long

/** * hdu2068 * 错排+组合数 */#include<iostream>#include<cstdio>using namespace std;const int mmax = 30;typedef long long ll;//巧妙求组合数ll C(int m, int n){    ll a = 1;    ll b = 1;    for(int i = 1; i <= m; i++, n--)    {        a *= n;        b *= i;    }    return a/b;}int main(){    int w[mmax];    w[0] = 1; w[1] = 0;    for(int i = 2; i <= 25; i++)    {        w[i] = (i-1)*(w[i-1] + w[i-2]);    }    int n;    while(scanf("%d", &n) && n)    {        ll s = 0;        for(int i = 0; i <= n/2; i++)        {            s+= w[i] * C(i, n);        }        printf("%lld\n", s);    }    return 0;}