[LeetCode]Merge k Sorted Lists

原题链接：http://oj.leetcode.com/problems/merge-k-sorted-lists/

题目描述：

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

题解：

　　这是个典型的分治题，用递归，不断平分，解决merge两个有序链表的小问题即可。

 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 *merge(ListNode* A,ListNode* B){
12         if(A==NULL && B==NULL)
13             return NULL;
14         if(A == NULL)
15             return B;
16         if(B == NULL)
17             return A;
18         ListNode *p = A;
19         ListNode *q = B;
20         ListNode *res = NULL;
21         ListNode *pre = NULL;
22         if(p->val<=q->val){
23             res = p;
24             p = p->next;
25             pre = res;
26         }else{
27             res = q;
28             q = q->next;
29             pre = res;
30         }
31         while(p!=NULL && q!=NULL){
32             if(p->val<=q->val){
33                 pre->next = p;
34                 pre = p;
35                 p = p->next;
36             }else{
37                 pre->next = q;
38                 pre = q;
39                 q = q->next;
40             }
41         }
42         while(p!=NULL){
43             pre->next = p;
44             pre = p;
45             p = p->next;
46         }
47         while(q!=NULL){
48             pre->next = q;
49             pre = q;
50             q = q->next;
51         }
52         pre->next = NULL;
53         return res;
54     }
55     
56     ListNode *mergeKLists(vector<ListNode *> &lists) {
57         int size = lists.size();
58         if(size == 0)
59             return NULL;
60         if(size == 1)
61             return lists[0];
62         vector<ListNode*> A,B;
63         for(int i=0; i<size/2; i++){
64             A.push_back(lists[i]);
65         }
66         for(int i=size/2; i<size; i++){
67             B.push_back(lists[i]);
68         }
69         return merge(mergeKLists(A),mergeKLists(B));
70     }
71 };