21. Merge Two Sorted Lists（python）

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.
题意：按顺序拼接两个已排序的链表
runtime:76ms
记住虚表头dummy设定好了千万不要动，设定一个p=dummy，然后两个表的指针一步步移动，并通过比较val的大小决定哪个插入p.next；最后返回dummy.next就可以获得正确的结果

# 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):
        if not l1:
            return l2
        if not l2:
            return l1
        dummy=ListNode(0)
        p=dummy
        while l1 and l2:
            if l1.valelse:
                p.next=l2
                l2=l2.next
            p=p.next
        if l1:
            p.next=l1
        else:
            p.next=l2
        return dummy.next