力扣HOT100 - 23. 合并K个升序链表

解题思路：

只要会合并两个升序链表，合并K个做法类似。

class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        ListNode res = null;
        for (int i = 0; i < lists.length; i++) {
            res = merge(res, lists[i]);
        }
        return res;
    }

    public ListNode merge(ListNode l1, ListNode l2) {
        if (l1 == null || l2 == null)
            return l1 == null ? l2 : l1;
        ListNode dum = new ListNode(0);
        ListNode cur = dum;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                cur.next = l1;
                l1 = l1.next;
            } else {
                cur.next = l2;
                l2 = l2.next;
            }
            cur = cur.next;
        }
        cur.next = l1 != null ? l1 : l2;
        return dum.next;
    }
}