合并两个有序链表

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]
示例 2：
    输入：l1 = [], l2 = []
    输出：[]
示例 3：
    输入：l1 = [], l2 = [0]
    输出：[0]

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/merge-two-sorted-lists
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

思路

Python：迭代（推荐）

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:

        dummy = ListNode()  # 伪头节点

        cur, l, r = dummy, list1, list2
        while l and r:
            if l.val < r.val:
                cur.next = l
                l = l.next
            else:
                cur.next = r
                r = r.next
            
            cur = cur.next
        
        cur.next = l or r  # 剩余部分
        return dummy.next

Python：递归

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:  # noqa
        if l1 is None:  # 尾递归 1
            return l2
        elif l2 is None:  # 尾递归 2
            return l1
        elif l1.val < l2.val:  # 选出头结点较小的一个，余下部分递归
            l1.next = self.mergeTwoLists(l1.next, l2)
            return l1
        else:
            l2.next = self.mergeTwoLists(l1, l2.next)
            return l2