Reverse Nodes In K Group

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Problem

Given the head of a linked list, reverse the nodes k at a time and return the modified list. If the number of remaining nodes is fewer than k, leave that final group as-is.

Examples

Example 1

Input:head = [1,2,3,4,5], k = 2

Output:[2,1,4,3,5]

Example 2

Input:head = [1,2,3,4,5], k = 3

Output:[3,2,1,4,5]

Constraints

1 <= k <= number of nodes <= 5000

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How to approach it: the Linked List pattern

A chain of nodes, each pointing to the next. No random access like an array, but insertion and deletion at a known position is O(1), and many classic problems come down to careful pointer rewiring.

Look for this pattern when

The problem explicitly gives you a linked list (reverse it, merge lists, detect a cycle, reorder it).
You need O(1) insertion/deletion without shifting elements, unlike an array.

Read the full Linked List guide →

Video walkthroughs

Original problem on LeetCode ↗