MergeKSortedLists.java

public class Solution {
    ListNode merge2Lists(ListNode list1, ListNode list2) {
        ListNode head    = new ListNode(-1);
        ListNode current = head;
        while(list1!=null&&list2!=null) {
            if(list1.val<list2.val) {
                current.next = list1;
                list1   = list1.next;
            } else {
                current.next = list2;
                list2   = list2.next;
            }
            current = current.next;
        }
        if(list1!=null) {
            current.next = list1;
        } else {
            current.next = list2;
        }
        return head.next;
    }
    public ListNode mergeKLists(List<ListNode> lists) {
        if(lists==null||lists.size()==0) {
            return null;
        }
        if(lists.size()==1) {
            return lists.get(0);
        }
        int length = lists.size() ;
        int mid = (length - 1)/2 ;
        ListNode l1 = mergeKLists(lists.subList(0,mid + 1)) ;
        ListNode l2 = mergeKLists(lists.subList(mid + 1,length)) ;

        return merge2Lists(l1,l2) ;
    }
}