21. Merge Two Sorted Lists

题目描述：
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

思路：
两链表不一定等长，先遍历比较两链表都不为空时的最小值存入新链表，当某一链表下一个值为空时，将另一链表剩余部分存入新链表

当前解决：

# Definition for singly-linked list.# class ListNode(object):#     def __init__(self, x):#         self.val = x#         self.next = Noneclass Solution(object):    def mergeTwoLists(self, l1, l2):        l3 = ListNode(0)        p = l3        while l1 and l2:            if l1.val < l2.val:                p.next = l1                l1 = l1.next            else:                p.next = l2                l2 = l2.next            p = p.next        if l1:            p.next = l1        else:            p.next = l2        return l3.next