剑指offer——74求1+2+3+n

题目描述

求1+2+3+...+n，要求不能使用乘除法、for、while、if、else、switch、case等关键字及条件判断语句（A?B:C）。

 

题解：
　　利用类的构造和析构

 //利用类的构造
 class Temp{
 public:
     Temp() { ++N; sum += N; }
     static void Reset() { N = ; sum = ; }
     static unsigned int getRes() { return sum; }
 private:
     static unsigned int N, sum;//一定得是静态，否则每次创建是会重新赋值
 };
 unsigned int Temp::N = ; //一定得先初始化
 unsigned int Temp::sum = ;

 class Solution {
 public:
     int Sum_Solution(int n) {
         Temp::Reset();
         Temp *t = new Temp[n];
         delete[]t;
         t = nullptr;
         return Temp::getRes();
     }
 };
 //利用类的析构
 class A;
 A *Array[];
 class A{
 public:
     virtual unsigned int sum(unsigned int n){
         return ;
     }
 };
 class B :public A {
 public:
     virtual unsigned int sum(unsigned int n) {
         return Array[!!n]->sum(n - ) + n;
     }
 };
 int getSum(int n) {
     A a;
     B b;
     Array[] = &a;
     Array[] = &b;
     return Array[]->sum(n);
 }
 //利用函数指针
 typedef unsigned int(*fun)(unsigned int);
 unsigned int Solution3_Teminator(unsigned int n){
     return ;
 }

 unsigned int Sum_Solution3(unsigned int n){
     static fun f[] = { Solution3_Teminator, Sum_Solution3 };
     return n + f[!!n](n - );
 }

 // 利用模板
 template <unsigned int n> struct Sum_Solution4{
     enum Value { N = Sum_Solution4<n - >::N + n };
 };

 template <> struct Sum_Solution4<>{
     enum Value { N =  };
 };

 template <> struct Sum_Solution4<>{
     enum Value { N =  };
 };