【HDOJ】2068 RPG的错排

其实就是排列组合+错排，得自学组合数学了。

#include <stdio.h>

#define MAXNUM 26

__int64 nums[MAXNUM] = {0,0,1};

int main() {
    int n;
    int i, j;
    __int64 tmp, k, res;

    for (i=3; i<MAXNUM; ++i) {
        nums[i] = (i-1) * (nums[i-1]+nums[i-2]);
    }
    while (scanf("%d", &n)!=EOF && n) {
        res = 1;
        for (i=2; i+i<=n; ++i) {
            // C^i_n
            k = tmp = 1;
            for (j=1; j<=i; ++j)
                k *= j;
            for (j=n; j>=n-i+1; --j)
                tmp *= j;
            res += tmp/k*nums[i];
        }
        printf("%I64d\n", res);
    }

    return 0;
}