2021-10-27 21. 合并两个有序链表【Easy】

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]

提示：

两个链表的节点数目范围是 [0, 50]
-100 <= Node.val <= 100
l1 和 l2 均按 非递减顺序 排列

方法一：

# 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:
        dummy = ListNode()
        curr = dummy
        while l1 and l2:
            if l1.val < l2.val:
                curr.next = l1
                l1 = l1.next
            else:
                curr.next = l2
                l2 = l2.next
            curr = curr.next

        if l1:
            curr.next = l1
        if l2:
            curr.next = l2

        return dummy.next

方法二：递归，待修正

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