"The sword refers to offer" asks 1+2+3+...+n

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Title Description
Ask for 1+2+3+...+n, the request cannot use multiplication method, for, while, if, else, switch, case and other keywords and conditional judgment statement (A? B:C).

Ideas
It seems like there is no good way to think about it at first, but if we are familiar with C + + features, or if we know more about recursion, we can find some ways to solve this problem.

1. Using recursion

Class Solution{public:int sum_solution (int n) {int Sum = N;bool flag = (n>0) && ((Sum+=sum_solution (n-1)) >0) ; return sum;}};

2. Constructors

Class temp{public    :        Temp ()        {            ++n;            sum+=n;        }        static void Reset ()        {            N = Sum = 0;        }        static int getsum ()        {            return Sum;        }    Private:        static int N;        static int Sum;}; int temp::n = 0;int temp::sum = 0; Class solution{public    :        int sum_solution (int n)        {            temp::reset ();            Temp *a = new Temp[n];            delete []a;            A = NULL;            return Temp::getsum ();        }};

3. Virtual functions

Class A; A * array[2]; Class a{public    :        virtual int Sum (int n)        {            return 0;        }}, Class B:public a{public    :        virtual int Sum (int n)        {            return array[!! N]->sum (n-1) +n;        }; Class solution{public    :        int sum_solution (int n)        {            a A;            b b;            Array[0] = &a;            ARRAY[1] = &b;            return Array[1]->sum (n);        }};

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"The sword refers to offer" asks 1+2+3+...+n