归并排序

 1 #include<iostream>
 2 #define MAX 5000003
 3 #define SENTINEL 2000000000
 4 using namespace std;
 5 
 6 int L[MAX / 2 + 2], R[MAX / 2 + 2];            
 7 int cnt;
 8 
 9 void merge (int A[],  int left, int mid, int right)  //合并 right为数组长 
10 {
11     int n1 = mid - left;
12     int n2 = right - mid;
13     for( int i = 0; i < n1; i++ )        L[i] = A[left + i];
14     for( int i = 0; i < n2; i++ )        R[i] = A[mid + i];
15     L[n1] = R[n2] = SENTINEL;                                 //分到最小时L、R中各有一个数，若进行比较合并，需比较两次，才能合并入A，故需要一个 很大的sentinel ； 
16     int i = 0, j = 0;
17     for(int k = left; k < right; k++)
18     {
19         cnt++;
20         if( L[i] <= R[j])
21             A[k] = L[i++];
22         else 
23             A[k] = R[j++];
24     }
25     
26 }
27 
28 void mergeSort( int A[], int left, int right)
29 {
30     if( left + 1 < right)
31     {
32         int mid = (left + right) / 2;
33         mergeSort(A, left, mid);
34         mergeSort(A, mid, right);
35         merge(A, left, mid,right);
36     }
37 }
38 
39 int main()
40 {
41     int A[MAX], n, i;
42     
43     cin >> n;
44     for( i = 0; i < n; i++)        cin >> A[i];
45     
46     mergeSort(A, 0, n);
47     
48     for( i = 0;i < n;i++ )
49     {
50         if( i ) cout << " ";
51         cout << A[i];    
52     } 
53     cout << endl;
54     
55     return 0;
56 }