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 = 0; sum = 0; }
    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 = 0; //一定得先初始化
unsigned int Temp::sum = 0;

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[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 getSum(int n) {
    A a;
    B b;
    Array[0] = &a;
    Array[1] = &b;
    return Array[1]->sum(n);
}
//利用函数指针
typedef unsigned int(*fun)(unsigned int);
unsigned int Solution3_Teminator(unsigned int n){
    return 0;
}

unsigned int Sum_Solution3(unsigned int n){
    static fun f[2] = { Solution3_Teminator, Sum_Solution3 };
    return n + f[!!n](n - 1);
}

// 利用模板
template <unsigned int n> struct Sum_Solution4{
    enum Value { N = Sum_Solution4<n - 1>::N + n };
};

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

template <> struct Sum_Solution4<0>{
    enum Value { N = 0 };
};