138. Copy List with Random Pointer

Copy List with Random Pointer (LeetCode 138) is a Medium Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Graph Traversal Patterns (DFS & BFS) pattern, in the Deep Copy / Cloning subpattern.

• NeetCode 150 · #40 of 150 · Linked List
• NeetCode 250 · #79 of 250 · Linked List

How to solve Copy List with Random Pointer

Copy List with Random Pointer is a Graph Deep Copy and Cloning problem, part of Graph Traversal Patterns (DFS & BFS). Learn to deep copy and clone graph structures using DFS/BFS with visited mapping.

Work towards the solution in these steps:

1. Consider how this problem can be viewed as cloning a complex graph where each node has multiple pointers, suggesting graph traversal techniques like DFS or BFS.
2. Think about how to keep track of already copied nodes to avoid duplicating nodes multiple times and to correctly assign the random pointers.
3. Using a hash map (or dictionary) can help map original nodes to their corresponding copied nodes, facilitating the deep copy process.

The reference solution runs in O(n) time and O(1) space.

The Copy List with Random Pointer solution and editorial walks through the full approach with reference code.

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, randomindex] where: val: an integer representing Node.val randomindex: 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:

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:

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]]

Example 4:

Input: head = [] Output: [] Explanation: The given linked list is empty (null pointer), so return null.

Constraints:

- 0 <= n <= 1000 - -10000 <= Node.val <= 10000 - Node.random is null or is pointing to some node in the linked list.

Topics and companies

Topics: Hash Table, Linked List.

Reported in interviews at Amazon, Facebook, Microsoft, Bloomberg, eBay, Oracle, Yahoo, Qualtrics.

How to practise Copy List with Random Pointer on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript, Kotlin, PHP, Python, python3, Ruby, Scala, Swift and TypeScript. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(n) time and O(1) space.

Open Copy List with Random Pointer in the code editor, or read the Copy List with Random Pointer editorial for a worked solution with its approach and complexity analysis.

Where Copy List with Random Pointer sits in the DSA pattern sheet

• Graph Traversal Patterns (DFS & BFS) pattern guide
• Deep Copy / Cloning subpattern
• The full DSA patterns sheet

Problems related to Copy List with Random Pointer

Other problems that use the same Deep Copy / Cloning and Graph Traversal Patterns (DFS & BFS) techniques:

• Clone Graph — Medium, same subpattern
• Clone N-ary Tree — Medium, same subpattern
• Find the City With the Smallest Number of Neighbors at a Threshold Distance — Medium, same subpattern
• Accounts Merge — Medium, same pattern
• Alien Dictionary — Hard, same pattern
• All Paths from Source Lead to Destination — Medium, same pattern

Copy List with Random Pointer FAQ