《剑指Offer（第二版）》面试题25. 合并两个排序的链表

输入两个递增排序的链表，合并这两个链表并使新链表中的节点仍然是递增排序的。

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

限制：
0 <= 链表长度 <= 1000

# 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
            
        if l1.val > l2.val:
            new = l2
            l2 = l2.next
            new.next = None
        else:
            new = l1
            l1 = l1.next
            new.next = None

        result = new
        while l1 and l2:
            if l1.val <=l2.val:
                flag = l1
                l1 = l1.next
                flag.next = None
                new.next = flag
                new = new.next
            else:
                flag = l2
                l2 = l2.next
                flag.next = None
                new.next = flag
                new = new.next
        if l1:
            new.next = l1
        if l2:
            new.next = l2

        return result