我正在尝试使用Python实现Skiplist。 你能帮助我吗？ 很简单

Question

This is just a very simple code and my reference is from here: http://www.mathcs.emory.edu/~cheung/Courses/323/Syllabus/Map/skip-list-impl.html#why-q

I think the insert function is okay but when I try to use the get() function, it doesn't return anything, instead it loops endlessly inside the searchEntry() part. I don't know what's wrong. In the insert() function, the searchEntry() operates well. It returns the reference to the floorEntry(k) entry containing a key that is smaller than the key that needs to be inserted in the skiplist. Please help me figure out the source of the error in the searchEntry() function. I'm sorry I'm not really good at this. Thank you!

from QuadLinkedList import QLLNode
import random
class Skippy:


    def __init__(self):

        self._p1 = QLLNode("MINUS_INF")
        self._p2 = QLLNode("PLUS_INF")

        self._head = self._p1
        self._tail = self._p2

        self._p1.setNext(self._p2)
        self._p2.setPrev(self._p1)

        self._height = 0
        self._n = 0

    def insert(self, key, value):

        p = self.searchEntry(key)
        print "p = " + str(p.getKey())
        q = QLLNode(key, value)
        q.setPrev(p)
        q.setNext(p.getNext())
        p.getNext().setPrev(q)
        p.setNext(q)

        i = 0

        while random.randint(0,1) != 0:

            if i >= self._height:
                self._height += 1

                newHead = QLLNode("MINUS_INF")
                newTail = QLLNode("PLUS_INF")

                newHead.setNext(newTail)
                newHead.setDown(self._head)

                newTail.setPrev(newHead)
                newTail.setDown(self._tail)

                self._head.setUp(newHead)
                self._tail.setUp(newTail)

                self._head = newHead
                self._tail = newTail

            while p.getUp() == None:
                p = p.getPrev()

            p = p.getUp()

            e = QLLNode(key,None)

            e.setPrev(p)
            e.setNext(p.getNext())
            e.setDown(q)

            p.getNext().setPrev(e)
            p.setNext(e)
            q.setUp(e)

            q = e

            i += 1

        self._n += 1
        return None

    def get(self, key):
        p = self.searchEntry(key)

        if key == p.getKey():
            return p.getElement()
        else:
            return "There's None!"

    def searchEntry(self, key):
        p = self._head

        while True:
            while p.getNext().getKey() != "PLUS_INF" and p.getNext().getKey() <= key:
                p = p.getNext()
            if p.getDown() != None:
                p = p.getDown()
            else:
                break
        return p

Answer 1

The issue isn't in the code for searchEntry , which appears to have the correct logic. The problem is that the list structure is getting messed up. I believe the issue is with the code you have for adding a new level to the list in insert . Specifically this bit:

       if i >= self._height:    #make an empty level

            self._height += 1

            self._minus.setNext(self._plus)
            self._minus.setDown(self._head)

            self._plus.setPrev(self._minus)
            self._plus.setDown(self._tail)

            self._head.setUp(self._minus)
            self._tail.setUp(self._plus)

            self._head = self._minus
            self._tail = self._plus

The thing that stands out to me about this code is that you're not creating any new nodes, just modifying existing ones, which is what is breaking your list structure. You need to create new head and tail nodes, I think, and link them into the top of the strucutre. ( minus and plus are not part of the algorithm as described at your link, so I'm not sure what you're doing with them.) Probably you want something like this:

       if i >= self._height:    #make an empty level

            self._height += 1

            newHead = QLLNode("MINUS_INF")
            newTail = QLLNode("PLUS_INF")

            newHead.setNext(newTail)
            newHead.setDown(self._head)

            newTail.setPrev(newHead)
            newTail.setDown(self._tail)

            self._head.setUp(newHead)
            self._head = newHead

            self._tail.setUp(newTail)
            self._tail = newTail