21.md

21. 合并两个有序链表

题目描述

将两个有序链表合并为一个新的有序链表并返回。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

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

Code

Solution1

迭代

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode head = dummy;
        while (l1 != null || l2 != null) {
            if (l1 == null) {
                head.next = new ListNode(l2.val);
                l2 = l2.next;
            } else if (l2 == null) {
                head.next = new ListNode(l1.val);
                l1 = l1.next;
            } else {
                if (l1.val >= l2.val){
                    head.next = new ListNode(l2.val);
                    l2 = l2.next;
                } else {
                    head.next = new ListNode(l1.val);
                    l1 = l1.next;
                }
            }
            head = head.next;
        }
        return dummy.next;
    }
}

Solution2

递归

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        if (l1.val < l2.val){
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }

    }
}

题解

https://leetcode-cn.com/problems/merge-two-sorted-lists/solution/he-bing-liang-ge-you-xu-lian-biao-by-leetcode/

递归：https://leetcode-cn.com/problems/merge-two-sorted-lists/solution/hua-jie-suan-fa-21-he-bing-liang-ge-you-xu-lian-bi/