merge K sorted LL

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
       ListNode* temp=NULL;
       ListNode* head=NULL;
       int k=lists.size();
       if(k==0){
           return NULL;
       }
       else if(k==1){
           head=lists[0];
           return head;
       }
       while(k>1){
           int min=INT_MAX;
           int tracker;
               for(int i=0;i<k;i++){
                   if(lists[i]==NULL){
                       lists[i]=lists[k-1]?lists[k-1]:NULL;
                       k--;
                       i--;                  
                   }                  
                    else if(lists[i]->val<min){
                       min=lists[i]->val;
                       tracker=i;
                   }
           }
           if(min==INT_MAX){
               return head;
           }
           lists[tracker]=lists[tracker]?lists[tracker]->next:NULL;
           ListNode* newNode=new ListNode(min);
           if(head==NULL){
           head=newNode;
           temp=newNode;
           }
           else{
           temp->next=newNode;
           temp=temp->next;
           }
       }
       temp->next=lists[0];
       return head;
    }
};