21. 合并两个有序链表

题目链接：https://leetcode-cn.com/problems/merge-two-sorted-lists/
难度：简单

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。

提示：
链表节点数目范围：[0, 50]

题解

思路：
没什么好说的

# 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()
        temp, p, q = dummy, list1, list2
        while p is not None and q is not None:
            if p.val < q.val:
                temp.next = p
                p = p.next
            else:
                temp.next = q
                q = q.next
            temp = temp.next
        temp.next = p if q is None else q
        return dummy.next

# 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]:
        if list1 is None:
            return list2
        if list2 is None:
            return list1
        if list1.val < list2.val:
            list1.next = self.mergeTwoLists(list1.next, list2)
            return list1
        else:
            list2.next = self.mergeTwoLists(list1, list2.next)
            return list2