【C++】primer plus 练习7.5

// n!#include<iostream>unsigned long count(int n);int main(){using namespace std;int n;unsigned long answer;cout <<"Enter a number(int).\nn=";cin >>n;if(cin >>n)                               //输入大于0；{answer =count(n);cout <<"n!="<<answer<<endl;}else cout <<"Error!\n";return 0;}unsigned long count(int n) //递归{unsigned long answer=0;if(n==1||n==0)answer =1;  else if(n>2)answer =n*count(n-1); elseanswer =answer+n*(n-1); // n=2时运算return answer;}