Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

题目大意：
将两个有序的链表合并为一个有序的链表

解题思路：
最容易想到的思路，边遍历边比较大小，将较小的值依次放到新建的结果链表中

# 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:
            
        if l1 == None:
            return l2
        if l2 == None:
            return l1
            
        head = ListNode(0)
        cur = head
        
        while l1 != None and l2 != None:
            if l1.val > l2.val:
                cur.next = l2
                l2 = l2.next
            else:
                cur.next = l1
                l1 = l1.next
            cur = cur.next

        if l1 == None:
            cur.next = l2

        if l2 == None:
            cur.next = l1
            
        return head.next