Copy List With Random Pointer

Problem

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

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.

Example 1:

image1

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:

image2

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

Example 3:

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

Constraints:

  • 0 <= n <= 1000

  • -10:sup:`4`<= Node.val <= 10:sup:`4`

  • Node.random is null or is pointing to some node in the linked list.

Pattern

Hash Table, Linked List

Approaches

Code

from __future__ import annotations

from collections import defaultdict


class Node:
    """Node in a linked list with a random pointer."""

    def __init__(
        self,
        x: int = 0,
        next: Node | None = None,
        random: Node | None = None,
    ):
        self.val = x
        self.next = next
        self.random = random


def copyRandomList(head: Node | None) -> Node | None:
    if head is None:
        return None

    old2copy: dict[Node | None, Node | None] = defaultdict(lambda: Node(0))
    old2copy[None] = None

    node = head
    copy_head = None
    while node:
        copy = old2copy[node]
        copy.val = node.val
        copy.next = old2copy[node.next]
        copy.random = old2copy[node.random]

        if copy_head is None:
            copy_head = copy

        node = node.next

    return copy_head

Test

>>> from copy_list_with_random_pointer__hash_map import copyRandomList, Node
>>> n1, n2, n3 = Node(7), Node(13), Node(11)
>>> n1.next, n2.next = n2, n3
>>> n2.random, n3.random = n1, n1
>>> c = copyRandomList(n1)
>>> [c.val, c.next.val, c.next.next.val]
[7, 13, 11]
>>> c is not n1
True
class copy_list_with_random_pointer__hash_map.Node(x: int = 0, next: Node | None = None, random: Node | None = None)

Bases: object

Node in a linked list with a random pointer.

copy_list_with_random_pointer__hash_map.copyRandomList(head: Node | None) → Node | None