merge k sorted lists

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

Example:

Input:
[
  1->4->5,
  1->3->4,
  2->6
]
Output: 1->1->2->3->4->4->5->6
*/

class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        ListNode * res;
        if(lists.size()==0) return NULL;
        else if(lists.size()==1){
            res=lists[0];
            return res;
        }
        else{
        ListNode * res=merge(lists[0], lists[1]);
        for(int i=2; i<lists.size(); i++){
           res=merge(res,lists[i]);
        }
            return res;
        }
//return res;
        
        
    }
private:
    ListNode* merge(ListNode *a, ListNode *b){
        if(a==NULL) return b;
        if(b==NULL) return a;
        ListNode *res;
        if(a->val<b->val){
            res=a;
            res->next=merge(a->next,b);
        }
        else{
            res=b;
            res->next=merge(a,b->next);
        }
return res;
    }
};