合并两个有序链表_21

21. 合并两个有序链表：https://leetcode-cn.com/problems/merge-two-sorted-lists/
将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。

class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode dummyHead = new ListNode(0);
        ListNode head = dummyHead;
        while (list1 != null && list2 != null) {
            if (list1.val < list2.val) {
                head.next = list1;
                list1 = list1.next;
            } else {
                head.next = list2;
                list2 = list2.next;
            }
            head =  head.next;
        }
        head.next = list1 == null ? list2 : list1;
        return dummyHead.next;
    }
}

23. 合并K个升序链表
给你一个链表数组，每个链表都已经按升序排列。
请你将所有链表合并到一个升序链表中，返回合并后的链表。

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

    public ListNode merge(ListNode[] lists, int l, int r){
        if(l == r) {     
            return lists[l];    
        }
        if(l > r)  {     
            return null;        
        }
        int mid = (l + r) >> 1;
        return mergerTwoLists(merge(lists, l, mid), merge(lists, mid+1, r));
    }

    public ListNode mergerTwoLists(ListNode l1, ListNode l2){
        ListNode dummy = new ListNode(0);
        ListNode prev = dummy;
        while(l1 != null && l2 != null) {
            int x = l1 == null ? 0: l1.val;
            int y = l2 == null ? 0: l2.val;
            if(x <= y){
                prev.next = l1;
                l1 = l1.next;
            } else {
                prev.next = l2;
                l2 = l2.next;
            }
            prev = prev.next;
        }
        prev.next = l1 == null ? l2: l1;
        return dummy.next;
    }
}