重复造轮子系列--插入排序和归并排序

Posted 2020-09-01 丹西

囧，道理很简单，实践起来却不容易。

因为编程语言跟算法描述数据结构并不能完全一致，所以理论到实践还是有些出入的。

下面的例子是没有哨兵位置的实现：

 1 #include <iostream>
 2 #include <vector>
 3 #include <algorithm>
 4 #include <cassert>
 5 
 6 using namespace std;
 7 
 8 template<typename _InIt, typename _Func>
 9 void insert_sort(_InIt first, _InIt last, _Func& Func) {
10     typedef typename iterator_traits<_InIt>::value_type _ValueType;
11 
12     for (_InIt it = first + 1; it != last; ++it) {
13         _ValueType key = *it;
14         _InIt ite = it - 1;
15         for (; (ite - first)>=0 && Func(*ite, key); --ite)
16             *(ite + 1) = *ite;
17         *(ite + 1) = key;
18     }
19 }
20 
21 template<typename _InIt, typename _Func>
22 void __merge(_InIt first, _InIt last, _Func& Func) {
23     typedef typename iterator_traits<_InIt>::value_type _ValueType;
24     vector<_ValueType> lv, rv;
25 
26     // if((last-first) == 1) return;  //这行代码折磨了我了好长时间，重新写了个C语言版找回自信，然后去洗了个澡回来发现了，赶紧去掉
27     size_t r = last-first;
28     size_t q = floor((r)/2);
29 
30     copy(first, first+q+1, back_inserter(lv));
31     copy(first+q+1, last+1, back_inserter(rv));
32 
33     typename vector<_ValueType>::iterator lit = lv.begin();
34     typename vector<_ValueType>::iterator rit = rv.begin();
35 
36     for (_InIt it=first; it!=last+1; ++it) {
37         if (Func(*lit, *rit)) {
38             if (lit != lv.end()) {
39                 *it = *lit;
40                 lit++;
41             }
42         } else {
43             if (rit != rv.end()) {
44                 *it = *rit;
45                 rit++;
46             }
47         }
48     }
49 }
50 
51 template<typename _InIt, typename _Func>
52 void __merge_sort(_InIt first, _InIt last, _Func& Func) {
53     if (last - first > 0) {
54         size_t q = floor((last - first)/2);
55         __merge_sort(first, first+q, Func);
56         __merge_sort(first+q+1, last, Func);
57         __merge(first, last, Func);
58     }
59 }
60 
61 template<typename _InIt, typename _Func>
62 void merge_sort(_InIt first, _InIt last, _Func& Func) {
63     __merge_sort(first, last-1, Func);
64 }
65 
66 template<typename T>
67 void print(const vector<T>& v) {
68     for(vector<int>::const_iterator it=v.begin(); it != v.end(); it++)
69            cout << *it << " ";
70         cout << endl;
71 }
72 
73 int main() {
74     int lst[] = {1,5,2,6,3,7,4,8};
75     vector<int> v(lst, lst+8);
76 
77     less<int> l;
78     insert_sort(v.begin(), v.end(), l);
79     print(v);
80 
81     int lst2[] = {2,4,5,7,1,2,3,6};
82     vector<int> v2(lst2, lst2+8);
83     less_equal<int> lq;
84 
85     merge_sort(v2.begin(), v2.end(), lq);
86     print(v2);
87 
88     return 0;
89 }