23. 合并K个升序链表

  class Solution {
        public ListNode mergeKLists(ListNode[] lists) {
            return merger(lists, 0, lists.length - 1);
        }

        private ListNode merger(ListNode[] lists, int l, int r) {
            if (l == r) {
                return lists[l];
            }
            if (l > r) {
                return null;
            }
            //右移一位相当于除2
            int mid=(l+r)>>1;

            return mergeTwoLists(merger(lists,l,mid),merger(lists,mid+1,r));
        }


        public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
            ListNode pre = new ListNode(0);
            ListNode curr = pre;
            while (l1 != null || l2 != null) {
                if (l1 == null) {
                    curr.next = l2;
                    return pre.next;
                }

                if (l2 == null) {
                    curr.next = l1;
                    return pre.next;
                }

                if (l1.val < l2.val) {
                    curr.next = l1;
                    curr = curr.next;
                    l1 = l1.next;
                } else {
                    curr.next = l2;
                    curr = curr.next;
                    l2 = l2.next;
                }
            }

            return pre.next;
        }
    }