劍指Offer——面試題64：求1+2+…+n

面試題64：求1+2+…+n

題目：求1+2+…+n，要求不能使用乘除法、for、while、if、else、switch、case 等關鍵字及條件判斷語句（A?B:C）。

🐱‍🏍解法一：利用構造函數求解

#include<iostream>
#include<algorithm>
#include<list>
using namespace std;
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();
}

int main(){
	printf("%d", Sum_Solution(10));
}

🐱‍🏍解法二：利用虛函數求解

#include<iostream>
#include<algorithm>
#include<list>
using namespace std;
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; // 當n不爲零時, 調用函數B::sum, 當n等於0時,調用函數 A::sum 
		}
};

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

int main(){
	printf("%d", Sum_Solution2(10));
}

🐱‍🏍解法三：利用函數指針求解

#include<iostream>
#include<algorithm>
#include<list>
using namespace std;
typedef unsigned int (*func)(unsigned int);

unsigned int Solution3_Teminator(unsigned int n){
	return 0;
}

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

int main(){
	printf("%d", Sum_Solution3(10));
}

🐱‍🏍解法四：利用模塊類型求解

#include<iostream>
#include<algorithm>
#include<list>
using namespace std;
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
	};
};

int main(){
	printf("%d", Sum_Solution4<10>::N);
}