HDOJ2049

HDACM2049
错排 + 组合

import java.util.Scanner;class Main{    public static void main(String args[]){        Scanner sc = new Scanner(System.in);        int c =sc.nextInt();        while(c-->0){            int n = sc.nextInt();            int m = sc.nextInt();            long sum1 =1;            long a=0,b=1;            long x=1;            long sum2 =n;            for (int i = n-m+1; i < n; i++) {                sum2 *= i;            }            for (int i = 1; i <= m; i++) {                sum1 *= i;                if (i==2) {                    x=1;                }                if (i>2) {                    x=(i-1)*(a+b);                    a=b;                    b=x;                }            }            System.out.println(sum2/sum1*x);//记得要先除后成先成的话会超出long的范围        }    }}