Copy List with Random Pointer
A medium Linked List problem included in Love Babbar 450, Striver A2Z. Below: the roles whose interviews prioritise this topic, and how to practise it.
- Topic
- Linked List
- Sheets
- 2
- Core for
- 3 roles
- Platform
- LeetCode
The problem
Deep copy a linked list where each node has a next pointer and an additional random pointer that could point to any node in the list or null.
Example 1
- Input
- [[7,null],[13,0],[11,4],[10,2],[1,0]]
- Output
- deep copy with same structure
Example 2
- Input
- [[1,1],[2,1]]
- Output
- deep copy with same structure
Example 3
- Input
- [[3,null]]
- Output
- deep copy of single node
Constraints
- 0 <= n <= 1000
- -10^6 <= Node.val <= 10^6
How to think about it
Updated 2026-09-09Copying nodes sequentially is simple, but random pointers can target nodes that have not been cloned yet. You must either keep a dictionary mapping original nodes to clones, or interweave clones directly beside originals so each clone finds its random target by looking at the original's next step.
Approaches, worst first
Hash map dictionary
time O(n) · space O(n)
Traverse the list once to instantiate a clone for every original node, storing mappings in a hash map `original -> clone`. Perform a second pass assigning `clone.next` and `clone.random` by looking up corresponding pointers in the map.
Interweaving nodes in placeWrite this one
time O(n) · space O(1)
Insert each cloned node directly after its original counterpart: `A -> A' -> B -> B'`. In a second pass, set `curr.next.random = curr.random.next`. In a third pass, decouple the interwoven chain into the original list and the clone list, achieving O(1) auxiliary space.
Where people lose marks · 3
- Dereferencing `curr.random.next` when `curr.random` is null triggers an unhandled null pointer exception.
- Failing to restore the original list's next pointers during decoupling mutates the input list, which breaks caller expectations.
- Handling an empty input list with `head == null` without an immediate guard causes initial property lookups to crash.
The theory behind it
Linked List — the ground this problem stands on. All Linked List problems
What Linked List is
A linked list is a chain of separate cargo cars connected by coupling hooks, scattered anywhere across memory rather than sitting in a tidy contiguous row. Each car, called a node, holds a single piece of data and a pointer directing traffic to the address of the next car in line. Because nodes connect only by directional links, jumping straight to the tenth car is impossible without walking past the first nine.
When to reach for it
Choose a linked list when a problem requires frequent insertions and deletions at known positions without shifting whole blocks of surrounding memory. Problems mentioning pointer splicing, reversing subsequences in place, merging sorted streams, or detecting cycles in linear chains strongly point here. It is ideal when total capacity is unpredictable and memory allocation must happen one individual node at a time.
How the pattern works
Think in terms of pointer rewiring before dereferencing. Keep a dummy head node pointing to the start of the list so modifications to the initial item do not require separate edge logic. Always save references to neighboring nodes into temporary variables before cutting or redirecting forward links. When diagnosing loops or locating middle nodes, advance two references simultaneously at differing velocities so traversal completes without supplementary storage.
What each operation costs
| Operation | Time |
|---|---|
| insert or delete at the head | O(1) |
| insert or delete after a known node | O(1) |
| find an element by value or position | O(n) |
What usually goes wrong with Linked List
- Losing access to the remainder of the chain by overwriting a next reference before caching the downstream node address in a temporary variable.
- Attempting to read properties of a null node reference after walking one step beyond the tail or advancing a fast runner without checking its next step.
- Creating an accidental infinite cycle by pointing a trailing node back into earlier segments of the chain without severing old outgoing links.
Which roles need this problem
Linked List is a core topic for these 3 roles — if you're targeting one of them, this problem is early in your path, not optional.
Secondary for 5 more roles, including Full-Stack Developer, Android Developer, iOS Developer.
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