Leetcode 23. Merge k Sorted Lists

 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* mergeTwoLists(ListNode* a,ListNode* b){
12         if(!a){
13             return b;
14         }
15         if(!b){
16             return a;
17         }
18         if(a->val>b->val){
19             b->next=mergeTwoLists(a,b->next);
20             return b;
21         }
22         a->next=mergeTwoLists(a->next,b);
23         return a;
24     }
25     ListNode* mergeKLists(vector<ListNode*>& lists) {
26         ListNode* head=NULL;//一定要初始化！！
27         while(lists.size()>1){
28             lists.push_back(mergeTwoLists(lists[0],lists[1]));
29             lists.erase(lists.begin());
30             lists.erase(lists.begin());
31         }
32         if(lists.size()){
33             head=lists[0];
34         }
35         return head;
36     }
37 };

 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* mergeTwoLists(ListNode* a,ListNode* b){
12         if(!a){
13             return b;
14         }
15         if(!b){
16             return a;
17         }
18         if(a->val>b->val){
19             b->next=mergeTwoLists(a,b->next);
20             return b;
21         }
22         a->next=mergeTwoLists(a->next,b);
23         return a;
24     }
25     ListNode* mergeKLists(vector<ListNode*>& lists) {
26         if(lists.empty()){
27             return NULL;
28         }
29         int len=lists.size();
30         int b=0,e=len-1;
31         while(b<e){
32             while(b<e){
33                lists[b]=mergeTwoLists(lists[b],lists[e]);
34                b++;
35                e--;
36             }
37             b=0;
38         }
39         return lists[0];
40     }
41 };

 1 class Solution {
 2 public:
 3     ListNode* mergeTwoLists(ListNode* a,ListNode* b){
 4         if(!a){
 5             return b;
 6         }
 7         if(!b){
 8             return a;
 9         }
10         if(a->val>b->val){
11             b->next=mergeTwoLists(a,b->next);
12             return b;
13         }
14         a->next=mergeTwoLists(a->next,b);
15         return a;
16     }
17     ListNode* mergeKLists(vector<ListNode*>& lists) {
18         vector<ListNode*> res;
19         int i;
20         for(i=0;i<lists.size();i++){
21             if(lists[i]){
22                 res.push_back(lists[i]);
23             }
24         }
25         if(!res.size()){
26             return NULL;
27         }
28         int len=res.size();
29         while(len>1){
30             //设len=2q或者2q+1
31             for(i=0;i<len/2;i++){//注意区分len是奇数还是偶数的情况
32                 res[i]=mergeTwoLists(res[i],res[len-1-i]);
33             }
34             //此时len应该等于q（len为偶数）或者q+1（len为奇数）
35             len=(len+1)/2;
36         }
37         return res[0];
38     }
39 };