leetcode21 合并两个有序链表 python实现

题目比较简单，写个文章记录一下

将两个有序链表合并为一个新的有序链表并返回。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

解答：

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

class Solution:
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        # 定义新链表的首结点
        head = ListNode(None)
        l3 = head
        
        while l1 != None and l2 != None:
            if l1.val >= l2.val:
                head.next = l2
                l2 = l2.next
            
            else:
                head.next = l1
                l1 = l1.next
            head = head.next
        if l1 != None :
            head.next = l1
        if l2 != None:
            head.next = l2
        return l3.next