要求: 合并两个有序链表并返回一个新的列表。新列表应该由连接在一起的节点前两个列表

    给定实例:

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution:
    def mergeTwoLists(self, l1, l2):
        if l1 is None:
            return l2
        if l2 is None:
            return l1
        pMerge = ListNode(None)
        if l1.val < l2.val:
            pMerge = l1
            pMerge.next = self.mergeTwoLists(l1.next, l2)
        else:
            pMerge = l2
            pMerge.next = self.mergeTwoLists(l1, l2.next)
        return pMerge