leetcode21合并两个有序链表

Posted 2021-09-13 lisin-lee-cooper

一.问题描述

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。

示例 1：

输入：l1 = [1,2,4], l2 = [1,3,4]
输出：[1,1,2,3,4,4]

二.示例代码

public class MergeTwoOrderedLinkList21 {

    public static void main(String[] args) {
        ListNode l1 = new ListNode(1);
        ListNode l3 = new ListNode(2);
        ListNode l5 = new ListNode(4);
        l1.next = l3;
        l3.next = l5;

        ListNode l2 = new ListNode(1);
        ListNode l4 = new ListNode(3);
        ListNode l6 = new ListNode(4);
        l2.next = l4;
        l4.next = l6;

        ListNode result = mergeTwoOrderedLinkList2(l1, l2);
        System.out.println(result);

    }

    private static ListNode mergeTwoOrderedLinkList(ListNode l1, ListNode l2) {

        ListNode result = new ListNode(0);
        ListNode pre = result;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                pre.next = new ListNode(l1.val);
                l1 = l1.next;
            } else {
                pre.next = new ListNode(l2.val);
                l2 = l2.next;
            }
            pre = pre.next;

        }

        pre.next = l1 == null ? l2 : l1;
        return result.next;
    }

    public static ListNode mergeTwoOrderedLinkList2(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        } else if (l2 == null) {
            return l1;
        } else if (l1.val < l2.val) {
            l1.next = mergeTwoOrderedLinkList2(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoOrderedLinkList2(l1, l2.next);
            return l2;
        }
    }

}