Deep copy list with random node

A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.

Return a deep copy of the list.

Note: Strategy is to do in O(n) time and use a hash to keep a node reference - O(n) space. The hash/dict has key as the original node and the value as the copy node. Later traverse the copied list and re-associate the random pointers by doing a hash lookup

# Definition for singly-linked list with a random pointer.
'''
class RandomListNode(object):
    def __init__(self, x):
        self.label = x
        self.next = None
        self.random = None
'''

class Solution(object):
    def copyRandomList(self, head):
        """
        :type head: RandomListNode
        :rtype: RandomListNode
        """
        #if not head:
            #return None

        nodeRefDict = {}

        ptr = head
        copyNodePrev = None
        copyNodeHead = None

        while ptr:
            #print 'ptr.label = ', ptr.label, ' ptr.random: ', ptr.random
            # create a copy of the node
            copyNode = RandomListNode(ptr.label)
            copyNode.random = ptr.random # keep an account of the original random node. Will replace this later

            # keep chaining the copyNode link list, when the previous node exists
            # so that we get a linked list for the copied nodes in the end
            if copyNodePrev:
                copyNodePrev.next = copyNode

            # nodeRefDict keeps an account between current node and the copy node
            # This will be used to re-associate the random node ptr in a separate iteration
            nodeRefDict[ptr] = copyNode 

            # do the following only 1 time, so that we can return the head of the copyNode list
            # this is just preserving the reference to the head of the copyNode list
            if copyNodeHead is None:
                copyNodeHead = copyNode

            copyNodePrev = copyNode
            ptr = ptr.next

        #////////////////////////////////////////////////////////////////////    
        # Traverse the copyNode list and re-associate the random pointers
        # to the correct copyNode items
        #////////////////////////////////////////////////////////////////////
        ptr = copyNodeHead
        while ptr:
            #print('copy ptr: ', ptr.label, ' copy ptr random: ', ptr.random)
            if ptr.random:
                ptr.random = nodeRefDict[ptr.random]
                #print('  new random: ', ptr.random.label)
            ptr = ptr.next

        return copyNodeHead