【LeetCode】21. 合并两个有序链表（JAVA）

原题地址：https://leetcode-cn.com/problems/merge-two-sorted-lists/

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：

代码：
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;
        }
    }
}

迭代：用了一个虚头节点

/**
 * 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 dummyHead = new ListNode(-1);
        ListNode cur = dummyHead;
        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;
        }
        if(l1 == null) cur.next = l2;
        if(l2 == null) cur.next = l1;
        return dummyHead.next;
    }
}