【LEETCODE OJ】Copy List with Random Pointer

Problem link:

http://oj.leetcode.com/problems/copy-list-with-random-pointer/

Deepcopy a linked list with random pointer, realy simple problem, solved in three steps using a hash map:

1. Create a new head, duplicating the head (return NULL for case head==NULL), map key=p1 with value=p2.
2. Let p1 point to head and p2 point to the new head. For each p1.next != NULL, duplicate p1.next and assign the new created node to p2.next, then move p1 and p2 to the next node and map them.
3. Scan each node of the old list again. For each node p, if p.random != None, set hash_map[p].random = hash_map[p.random].

The following code is my python solution accepted by oj.leetcode.

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

class Solution:
    # @param head, a RandomListNode
    # @return a RandomListNode
    def copyRandomList(self, head):
        if head is None:
            return None
        mapping = {}
        # Duplicate the head
        new_head = RandomListNode(head.label)
        # Duplicate the node and next pointer
        p1 = head
        p2 = new_head
        mapping[p1] = p2
        while p1.next is not None:
            p2.next = RandomListNode(p1.next.label)
            p2 = p2.next
            p1 = p1.next
            mapping[p1] = p2
        # Duplicate the random pointer
        p1 = head
        while p1 is not None:
            r1 = p1.random
            if r1 is not None:
                mapping[p1].random = mapping[r1]
            p1 = p1.next
        # Return
        return new_head