2020-04-21 No.21 合并两个有序链表

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：
输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

题解：一个为None后即可结束，剩下的补充上

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

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        dummy = ListNode(-1)
        head = dummy
        while l1 and l2:
            if l1.val <= l2.val:
                head.next = ListNode(l1.val)
                head = head.next
                l1 = l1.next
            else:
                head.next = ListNode(l2.val)
                head = head.next
                l2 = l2.next
        head.next = l1 if l1 else l2
        return dummy.next