力扣-23-合并K个排序链表

java:
public ListNode mergeKLists(ListNode[] lists) {
    if(lists == null || lists.length == 0 )    return null;
    ListNode head = lists[0];
    for(int i = 1;i<lists.length;i++){
        ListNode node = lists[i];
        head = merge(head,node);
    }
    return head;
}

private ListNode merge(ListNode l1,ListNode l2){
    if(l1 == null)  return l2;
    if(l2 == null)  return l1;
    if(l1.val <= l2.val){
        l1.next = merge(l1.next,l2);
        return l1;
    }else{
        l2.next = merge(l1,l2.next);
        return l2;
    }
}