Sort List

Sort a linked list in O(n log n) time using constant space complexity.

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode *sortList( ListNode *head )
12     {
13         vector<int> vec;
14         ListNode *p = head;
15         while( p != NULL )
16         {
17             vec.push_back( p->val );
18             p = p->next;
19         }
20         
21         vector<int> tmp( vec.size() );
22         
23         mergeSort( vec, tmp, 0, vec.size() - 1 );
24         
25         p = head;
26         for(vector<int>::iterator it = vec.begin(); it != vec.end(); it++, p = p->next)
27             p->val = *it;
28             
29         return head;
30     }
31     
32     void mergeSort( vector<int> &vec, vector<int> &tmp, int left, int right )
33     {
34         if( left < right )
35         {
36             int center = ( left + right ) / 2;
37             mergeSort( vec, tmp, left, center );
38             mergeSort( vec, tmp, center + 1, right );
39             merge( vec, tmp, left, center + 1, right );
40         }
41     }
42     
43     void merge( vector<int> &vec, vector<int> &tmp, int leftPos, int rightPos, int rightEnd )
44     {
45         int leftEnd = rightPos - 1;
46         int tmpPos = leftPos;
47         int numElements = rightEnd - leftPos + 1;
48         
49         while(leftPos <= leftEnd && rightPos <= rightEnd)
50             if(vec[leftPos] < vec[rightPos])
51                 tmp[tmpPos++] = vec[leftPos++];
52             else
53                 tmp[tmpPos++] = vec[rightPos++];
54                 
55         while(leftPos <= leftEnd)
56             tmp[tmpPos++] = vec[leftPos++];
57             
58         while(rightPos <= rightEnd)
59             tmp[tmpPos++] = vec[rightPos++];
60             
61         for(int i = 0; i < numElements; i++, rightEnd--)
62             vec[rightEnd] = tmp[rightEnd];
63     }
64 };

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原文地址：https://www.cnblogs.com/YQCblog/p/3970214.html