#21. 合并两个有序链表

题目：

 思路：这个题目比较简单，有点类似升序数组。

代码：

# 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:
        head = ListNode(0) #定义一个头结点
        c1=head #让c1等于头结点，通过操作c1，最后返回头结点的后继节点
        if not l1:#临界问题
            return l2
        if not l2 :
            return l1

        while l1 and l2:
            if l1.val <= l2.val:
                c1.next = l1
                l1=l1.next
            else:
                c1.next = l2
                l2=l2.next
            c1=c1.next
        if l1:
            c1.next = l1
        if l2:
            c1.next = l2
        return head.next