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

 1 //利用类的构造
 2 class Temp{
 3 public:
 4     Temp() { ++N; sum += N; }
 5     static void Reset() { N = 0; sum = 0; }
 6     static unsigned int getRes() { return sum; }
 7 private:
 8     static unsigned int N, sum;//一定得是静态，否则每次创建是会重新赋值
 9 };
10 unsigned int Temp::N = 0; //一定得先初始化
11 unsigned int Temp::sum = 0;
12 
13 class Solution {
14 public:
15     int Sum_Solution(int n) {
16         Temp::Reset();
17         Temp *t = new Temp[n];
18         delete[]t;
19         t = nullptr;
20         return Temp::getRes();
21     }
22 };
23 //利用类的析构
24 class A;
25 A *Array[2];
26 class A{
27 public:
28     virtual unsigned int sum(unsigned int n){
29         return 0;
30     }
31 };
32 class B :public A {
33 public:
34     virtual unsigned int sum(unsigned int n) {
35         return Array[!!n]->sum(n - 1) + n;
36     }
37 };
38 int getSum(int n) {
39     A a;
40     B b;
41     Array[0] = &a;
42     Array[1] = &b;
43     return Array[1]->sum(n);
44 }
45 //利用函数指针
46 typedef unsigned int(*fun)(unsigned int);
47 unsigned int Solution3_Teminator(unsigned int n){
48     return 0;
49 }
50 
51 unsigned int Sum_Solution3(unsigned int n){
52     static fun f[2] = { Solution3_Teminator, Sum_Solution3 };
53     return n + f[!!n](n - 1);
54 }
55 
56 // 利用模板
57 template <unsigned int n> struct Sum_Solution4{
58     enum Value { N = Sum_Solution4<n - 1>::N + n };
59 };
60 
61 template <> struct Sum_Solution4<1>{
62     enum Value { N = 1 };
63 };
64 
65 template <> struct Sum_Solution4<0>{
66     enum Value { N = 0 };
67 };