LeetCode 21. Merge Two Sorted Lists--合并2个有序列表--python递归，迭代解法

题目地址：

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

这道题比较简单，有多种解法。

python递归解法如下：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = Noneclass Solution:def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:if l1 is None:return l2elif l2 is None:return l1elif l1.val < l2.val:l1.next = self.mergeTwoLists(l1.next, l2)return l1else:l2.next = self.mergeTwoLists(l1, l2.next)return l2

Python迭代解法：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = Noneclass Solution:def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:prehead = ListNode(-1)prev = preheadwhile l1 and l2:if l1.val <= l2.val:prev.next = l1l1 = l1.nextelse:prev.next = l2l2 = l2.next            prev = prev.next# exactly one of l1 and l2 can be non-null at this point, so connect# the non-null list to the end of the merged list.prev.next = l1 if l1 is not None else l2return prehead.next

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