[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。 
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#  示例 1： 
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# 输入：l1 = [1,2,4], l2 = [1,3,4]
# 输出：[1,1,2,3,4,4]
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#  示例 2： 
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# 输入：l1 = [], l2 = []
# 输出：[]
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#  示例 3： 
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# 输入：l1 = [], l2 = [0]
# 输出：[0]
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#  提示： 
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#  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 
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#  Related Topics 递归 链表 👍 1974 👎 0


# leetcode submit region begin(Prohibit modification and deletion)
# 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(-1)
        pointer = dummy
        while l1 and l2:
            if l1.val <= l2.val:
                pointer.next = l1
                pointer, l1 = l1, l1.next
            else:
                pointer.next = l2
                pointer, l2 = l2, l2.next
        if l1:
            pointer.next = l1
        if l2:
            pointer.next = l2
        return dummy.next
# leetcode submit region end(Prohibit modification and deletion)