【剑指Offer】求1+2+3+...+n

转载于《剑指Offer》

• 题目：
  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）。
• 用析构函数求解：

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

• 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;
};

• 利用函数指针求解

typedef unsigned int(*fun)(unsigned int);
unsigned int Solution_teminator(unsigned int n){
    return 0;
}
unsigned int Sum_Solution(unsigned int n){
    static fun f[2] = {Solution_teminator, Sum_Solution};
    return n + f[!!n](n - 1);
}