leetcode-Merge k Sorted Lists-23（未完待续）

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* merge(ListNode* l1,ListNode* l2){
if(l1==NULL) return l2;
if(l2==NULL) return l1;
ListNode* head=NULL;
ListNode* tail=NULL;
ListNode* p=l1;
ListNode* q=l2;
if(p->val <= q->val){
head=p;
tail=p;
p=p->next;
tail->next=NULL;
}
else{
head=q;
tail=q;
q=q->next;
tail->next=NULL;
}
while(p&&q){
if(p->val <= q->val){
tail->next=p;
tail=p;
p=p->next;
tail->next=NULL;
}
else{
tail->next=q;
tail=q;
q=q->next;
tail->next=NULL;
}
}
if(p) tail->next=p;
else tail->next=q;
return head;
}
ListNode* mergeKLists(vector<ListNode*>& lists) {
if(lists.size()==0) return NULL;
ListNode* head=lists[0];
for(int i=1;i<lists.size();i++){
head=merge(head,lists[i]);
}
return head;
}
};