Reorder List
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
Reorder a singly linked list by interleaving the first half with the reversed second half.
Example 1
- Input
- [1,2,3,4]
- Output
- [1,4,2,3]
Example 2
- Input
- [1,2,3,4,5]
- Output
- [1,5,2,4,3]
Example 3
- Input
- [1,2]
- Output
- [1,2]
Constraints
- 1 <= n <= 5*10^4
How to think about it
Updated 2026-09-09The final sequence zips together the first half reading forward and the second half reading backward. Singly linked lists cannot step backwards, so you must split the list at its midpoint, reverse the second half in place, and splice the two halves node by node.
Approaches, worst first
Index array buffer
time O(n) · space O(n)
Collect every node reference into an indexed array, then wire pointers using two pointers converging from opposite ends. Simple to conceptualize, but consumes linear auxiliary space where in-place pointer manipulation suffices.
Find middle, reverse half, interleaveWrite this one
time O(n) · space O(1)
Use fast and slow pointers to locate the middle node and disconnect the second half. Reverse that second half iteratively. Interleave the two lists by stepping forward and weaving pointers alternately between the first and second chains.
Where people lose marks · 3
- Failing to set the middle node's next pointer to null leaves the first half connected to the reversed half, introducing a cycle at the tail of the reordered list.
- Splitting odd-length lists at the wrong middle node creates uneven halves that lead to dangling unlinked nodes or premature null dereferences during interleaving.
- Lists with 1 or 2 nodes need no reordering and can trigger null reference errors if the runner expects at least three elements.
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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