Data structures: merge sort

The two ordered series combined into an ordered series, which we call " merge ."
Merge sort (Merge Sort) is to use the idea of merging the number of columns to sort. According to a particular implementation, merge sort including " top-down " and " bottom-up " in two ways.

1.  From the bottom up merge sort : to be sorted columns divided into a number of length sub-number of columns, and these columns twenty-two combined; obtained several one ordered sequence 2, then the number of columns twenty-two They were combined; to obtain a plurality of ordered sequence length of 4, then they were combined twenty-two; incorporated directly up into a number of columns. This was the sort of result we want. (Refer to picture below)

2.  From top to bottom merge sort : it and "bottom-up" in the opposite direction of the sort. It basically comprises three steps:
① Decomposition - The current interval is divided into two, i.e., seeking split point = MID (High + Low) / 2; 
② Solving - recursively two subintervals a [low ... mid] and a [mid + 1 ... high] for merge sort. Recursive termination condition is the length of a sub-interval.
③ combined - is_sorted two subintervals a [low ... mid] and a [mid + 1 ... high] as an ordered merge interval a [low ... high].

 

 Merge sort down from the implementation code:

 1 //归并排序
 2 #include <iostream>
 3 #include <cstdio>
 4 #include <cstring>
 5 #include <string>
 6 #include <cmath>
 7 #include <algorithm>
 8 using namespace std;
 9 void Merge(int a[],int start,int mid,int end)
10 {
11     int *temp=new int[end-start+1];
12     int i=start;
13     int j=mid+1;
14     int k=0;
15     while(i<=mid&&j<=end)
16     {
17         if(a[i]<=a[j]) temp[k++]=a[i++];
18         else temp[k++]=a[j++];
19     }
20     while(i<=mid) temp[k++]=a[i++];
21     while(j<=end) temp[k++]=a[j++];
22     for(i=0;i<k;i++) a[start+i]=temp[i];
23     delete[] temp;
24 }
25 void MergeSort_Down(int a[],int start,int end)
26 {
27     if(a==NULL||start>=end) return;
28     int mid=(start+end)/2;
29     MergeSort_Down(a,start,mid);
30     MergeSort_Down(a,mid+1,end);
31     Merge(a,start,mid,end); 
32 }
33 int main()
34 {
35     int a[]={999,888,333,221,345,990,899};
36     int ilen=sizeof(a)/sizeof(a[0]);
37     cout<<"排序前:"<<endl;
38     for(int i=0;i<ilen;i++)cout<<a[i]<<" ";
39     cout<<endl;
40     MergeSort_Down(a,0,ilen-1);
41     cout<<"排序后:"<<endl;
42     for(int i=0;i<ilen;i++) cout<<a[i]<<" ";
43     cout<<endl;
44     return 0; 
45  } 

 

 Reference: https://www.cnblogs.com/skywang12345/p/3602369.html