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

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *mergeKLists(vector<ListNode *> &lists) 
    {
        int n = lists.size();
        if(n == 0 )
            return NULL;
        if(n == 1)
            return lists[0];
        while(n!=1)
        {
            vector<ListNode*> list_temp;
            for(int i=0; i<n; i=i+2)
            {
                int end = i+1;
                if(i+1>=n)
                    end = n-1;
                if(end == i)
                {
                    list_temp.push_back(lists[i]);
                    break;
                }
                ListNode* temp = mergeTwoLists(lists[i],lists[end]);
                list_temp.push_back(temp);
            }
            lists = list_temp;
            n = lists.size();
        }
        return lists[0];
    }
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) 
    {
        if(l1 == NULL)
            return l2;
        if(l2 == NULL)
            return l1;
        ListNode* head;
        ListNode* temp;
        if(l1->val < l2->val)
        {
            head = l1;
            temp = l2;
        }
        else
        {
            head = l2;
            temp = l1;
        }
        ListNode* temp1 = head->next;
        ListNode* now = head;
        while(temp1!=NULL && temp!=NULL)
        {
            if(temp1->val > temp->val)
            {
                now->next = temp;
                now = now->next;
                temp = temp->next;
            }
            else
            {
                now->next = temp1;
                now = now->next;
                temp1 = temp1->next;
            }
        }
        if(temp1 == NULL)
            now->next = temp;
        if(temp == NULL)
            now->next = temp1;
        return head;
    }
};

LeetCode--Merge k Sorted Lists