1+2+3+...+n不能用while、for、if else等实现

#include <iostream>  
using namespace std; 
class Plus 
{ 
 public: 
    Plus() { ++i, sum += i; } 
    //去掉下面的reset函数试试 
    static void reset() { i = 0; sum = 0; } 
    //去掉下面的static关键字试试 
    static int getSum() { return sum; } 
 
private: 
   static int i; 
   static int sum;  
}; 
 
int Plus::i = 0; 
int Plus::sum = 0; 
  
int cacSum(int n) 
 { 
    //只有static函数才能如此调用 
    Plus::reset();  
  
     Plus *test = new Plus[n]; 
    delete[] test; 
 
    return Plus::getSum(); 
 } 
  
 int main() 
 { 
    int i = 100; 
    int result = cacSum(i); 
    cout << "result = " << result << endl; 
	system("pause");
	return 0;
 }

方法2

int getSum(int n)
 {
	 int result=0;
	 (n>0)&&(result=getSum(n-1)+n);
	 return result;
 }