Leetcode Merge k Sorted Lists

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeKLists(List<ListNode> lists) {
        if(lists == null || lists.size() == 0){
            return null;
        }
        return mergeKLists(lists,0,lists.size() - 1);
    }
    
    private ListNode mergeKLists(List<ListNode> lists,int start,int end){
        if(start == end){
            return lists.get(start);
        }
        
        if((end - start) == 1){
            return mergeTwoLists(lists.get(start),lists.get(end));
        }
        
        ListNode left = mergeKLists(lists,start,(start+end)/2);
        ListNode right = mergeKLists(lists,(start+end)/2+1,end);
        return mergeTwoLists(left,right);
    }
    
    private ListNode mergeTwoLists(ListNode first,ListNode second){
        if(first == null){
            return second;
        }
        
        if(second == null){
            return first;
        }
        
        ListNode ret = null;
        
        if(first.val <= second.val){
            ret = first;
            first = first.next;
        }else{
            ret = second;
            second = second.next;
        }
        
        ListNode insert = ret;

        while(first!=null && second!=null){
            if(first.val <= second.val){
                insert.next = first;
                first = first.next;
            }else{
                insert.next = second;
                second = second.next;
            }
            
            insert = insert.next;
            insert.next= null;
        }
        
        insert.next = (first == null ? second:first);
        return ret;
    }
}