# Definition for singly-linked list.
# class ListNode(object):
#   def __init__(self, x):
#     self.val = x
#     self.next = None
class Solution(object):
  def mergeTwoLists(self, l1, l2):
    """
    :type l1: ListNode
    :type l2: ListNode
    :rtype: ListNode
    """
    #先考虑链表其中一个为空的情况
    if not l1:
      return l2
    if not l2:
      return l1
    curNode1 = l1
    curNode2 = l2
    #先选出第一个节点
    if curNode1.val < curNode2.val:
      head = curNode1
      curNode1 = curNode1.next
    else:
      head = curNode2
      curNode2 = curNode2.next
    cur = head
    while curNode1 and curNode2:
      if curNode1.val < curNode2.val:
        cur.next = curNode1
        curNode1 = curNode1.next
      else:
        cur.next = curNode2
        curNode2 = curNode2.next
      cur = cur.next
    #一直循环到有一个链表先结束
    #如果是链表1先结束，则拼上链表2剩余的那段
    if not curNode1:
      cur.next = curNode2
    #如果是链表2先结束，则拼上链表1剩余的那段
    else:
      cur.next = curNode1
    return head