求1+2＋．．．＋n

不用if.else.while等一切的条件判断

这题真是涨知识了：

１．　

#include<stdio.h>
typedef int (*fun)(int);
int f1(int i)
{
    return 0;
}
int f2(int i)
{
    fun f[2]={ f1,f2 };
    return i+f[!!i](i-1);
}
int main()
{
    int sum = f2(100);
    printf("%d
",sum);
    return 0;
}

２．　运用c++的static性质

class Temp
{
public:
      Temp() { ++ N; Sum += N; }

      static void Reset() { N = 0; Sum = 0; }
      static int GetSum() { return Sum; }

private:
      static int N;
      static int Sum;
};

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

int solution1_Sum(int n)
{
      Temp::Reset();

      Temp *a = new Temp[n];
      delete []a;
      a = 0;

      return Temp::GetSum();
}

３．

template <int n> struct solution4_Sum
{
      enum Value { N = solution4_Sum<n - 1>::N + n};
};


template <> struct solution4_Sum<1>
{
      enum Value { N = 1};
};

转自

http://zhedahht.blog.163.com/blog/static/2541117420072915131422/