21. 合并两个有序链表
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
# 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
"""
isFirst = True
if l1==None:
return l2
if l2 == None:
return l1
while l2!=None:
if isFirst:
if l1.val >= l2.val:
c = ListNode(l2.val)
c.next = l1
l1 = c
l2 = l2.next
n = l1
isFirst = False
elif l1.next==None:
l1.next = l2
return n
else:
if l1.next.val < l2.val:
l1 = l1.next
else:
c = ListNode(l2.val)
c.next = l1.next
l1.next = c
l2 = l2.next
return n
