Practice Leetcode 21. 合并两个有序链表

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：

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

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/merge-two-sorted-lists

思路1：

class Solution: def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode: if l1 is None: return l2 elif l2 is None: return l1 elif l1.val < l2.val: l1.next = self.mergeTwoLists(l1.next, l2) return l1 else: l2.next = self.mergeTwoLists(l1, l2.next) return l2

思路2：

class Solution: def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode: prehead = ListNode(-1) pre = prehead while l1 and l2: if l1.val <= l2.val: pre.next = l1 l1 = l1.next else: pre.next = l2 l2 = l2.next pre = pre.next pre.next = l1 if l1 else l2 return prehead.next