offer_64

//求1+2+...+n
//解法1：利用构造函数求解
class Temp
{
  public:
    Temp(){++N;Sum+=N;}
    static void Reset(){N=0;Sum=0;}
    static Unsigned int GetSum(){return Sum;}
    
  private:
    static unsigned int N;
    static unsigned int Sum;
};

unsigned int Temp::N=0;
unsigned int Temp::Sum=0;

unsigned int Sum_Solution1(unsigned int n)
{
  Temp::Reset();
  Temp *a=new Temp[n];
  delete []a;
  a=nullptr;
  
  return Temp::GetSum;
}

//解法2：利用虚函数求解
Class A;
A* Array[2];

class A
{
  public:
    virtual unsigned int Sum(unsigned int n)
    {
      return 0;
    }
};

class B:public A
{
  public:
    virtual unsigned int Sum(unsigned int n)
    {
      return Array[!!n]->sum(n-1)+n;
    }
};

int Sum_Solution2(int n)
{
  A a;
  B b;
  Array[0]=&a;
  Array[1]=&b;
  
  int value=Array[1]->Sum(n);
  
  return value;
}

//解法3：利用函数指针求解
typedef unsigned int (*fun)(unsigned int);
unsigned int Solution3_Teminator(unsigned int n)
{
  return 0;
}

unsigned int Sum_Soultion3(unsigned int n)
{
  static fun f[2]={Solution_Teminator,Sum_Solution3};
  return n+f[!!n](n-1);
}


//解法4：利用模板类型求解
template<unsigned int n>
struct Sum_Solution4
{
  enum Value{N=Sum_Solution4<n-1>::N-1};
}

template<>struct Sum_Solution4<1>
{
  enum Value{N=1};
}