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Copy List with Random Pointer

Problem Statement​

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

Construct a deep copy of the list. The deep copy should consist of exactly n brand new nodes, where each new node has its value set to the value of its corresponding original node. Both the next and random pointer of the new nodes should point to new nodes in the copied list such that the pointers in the original list and copied list represent the same list state. None of the pointers in the new list should point to nodes in the original list.

For example, if there are two nodes X and Y in the original list, where X.random --> Y, then for the corresponding two nodes x and y in the copied list, x.random --> y.

Return the head of the copied linked list.

The linked list is represented in the input/output as a list of n nodes. Each node is represented as a pair of [val, random_index] where:

  • val: an integer representing Node.val
  • random_index: the index of the node (range from 0 to n-1) that the random pointer points to, or null if it does not point to any node. Your code will only be given the head of the original linked list.

LeetCode link

Example 1:​

alt text

Input: head = [[7,null],[13,0],[11,4],[10,2],[1,0]]
Output: [[7,null],[13,0],[11,4],[10,2],[1,0]]

Example 2:​

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Input: head = [[1,1],[2,1]]
Output: [[1,1],[2,1]]

Example 3:​

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Input: head = [[3,null],[3,0],[3,null]]
Output: [[3,null],[3,0],[3,null]]

Example 3:​

Input: head = []
Output: []

The given linked list is empty (null pointer), so return null.

Constraints:​

  • 0 <= n <= 1000
  • -104 <= Node.val <= 104
  • Node.random is null or is pointing to some node in the linked list.

Implementation​

We will be using two passes method, where in the first pass we make copy of the nodes and store it in hashmap. In second pass we link them. We use two passes as this cannot be done in single pass because there might be a case where we have to link the node which isn't created yet while copying.

  • Space Complexity: O(n)
  • Time Complexity: O(n)

Code​

Python Code
"""
# Definition for a Node.
class linkedListNode:
    def __init__(self, value: int, nextNode: 'linkedListNode' = None, random: 'linkedListNode' = None):
        self.value = value
        self.nextNode = nextNode
"""
class Solution:
    def copyRandomList(self, head: 'linkedListNode'):
        if head is None:
            return head

        # when the element has to point null
        hashmap = {None: None}

        curr = head
        # copy the nodes to hashmap
        while curr != None:
            newNode = linkedListNode(curr.value + '@')
            hashmap[curr] = newNode
            curr = curr.nextNode

        curr = head
        # link the nodes
        while curr != None:
            newNode = hashmap[curr]
            newNode.nextNode = hashmap[curr.nextNode]
            newNode.random = hashmap[curr.random]
            curr = curr.nextNode

        return hashmap[head]