用三种循环结构实现20！.

#include<iostream>
using namespace std;
int main()
{
int b=1;
for (int i=1; i<=20; i++){

b=b*i;
}
cout << "20的阶层为：" << b << endl;
system("pause");
return 0;
}

#include <iostream>
using namespace std;
int main ()
{
int N;
cin>>N;
int i;
int cur=1;
long double sum=1;
for(i=0;i<N;i++)
{
sum*=cur;
cur++;
}
cout<<sum;
cout<<endl;
sum=1;
cur=1;
do
{
++cur;
sum*=cur;
}while(cur<N);
cout<<sum;
cout<<endl;
sum=1;
cur=1;
while(cur<=N)
{
sum*=cur;
cur++;
}
cout<<sum;
cout<<endl;
return 0;
}
写的算是比较清楚了 如果还有问题 我再帮你看看

那我再写一个Do_while吧：
#include<stdio.h>
void main()
{