leetcode 【 Merge Two Sorted Lists 】 python 实现

题目：

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.

代码：oj在线测试通过 Runtime: 208 ms

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param two ListNodes
    # @return a ListNode
    def mergeTwoLists(self, l1, l2):
        if l1 is None:
            return l2
        if l2 is None:
            return l1
        
        dummyhead = ListNode(0)
        dummyhead.next = None
        p = dummyhead
        while l1 is not None and l2 is not None:
            if l1.val > l2.val:
                p.next = l2
                l2 = l2.next
            else:
                p.next = l1
                l1 = l1.next
            p = p.next
        if l1 is not None:
            p.next = l1
        else:
            p.next = l2
        return dummyhead.next

思路：

虚表头dummyhead设定好了就不要动，设定一个p=hummyhead，然后从处理p.next开始；最后返回hummyhead就可以获得正确的结果

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

注意，每次执行完判断，要移动指针p=p.next