[leetcode] 21. Merge Two Sorted Lists

题目

• You are given the heads of two sorted linked lists list1 and list2.

  Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

  Return the head of the merged linked list.

  Example 1:

Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2:

Input: list1 = [], list2 = []
Output: []

Example 3:

Input: list1 = [], list2 = [0]
Output: [0]

Constraints:

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both list1 and list2 are sorted in non-decreasing order.

思路

递归

代码

python版本：

# 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 not list1 and not list2:
            return None
        root=ListNode()
        if not list1:
            root.val=list2.val
            root.next=self.mergeTwoLists(list1,list2.next)
        elif not list2:
            root.val=list1.val
            root.next=self.mergeTwoLists(list1.next,list2)
        elif list1.val<list2.val:
            root.val=list1.val
            root.next=self.mergeTwoLists(list1.next,list2)
        else:
            root.val=list2.val
            root.next=self.mergeTwoLists(list1,list2.next)
        return root