DSA · INTERVIEW & OA PREP

Topic 3 of 12

Linked Lists, Stacks & Queues

Pointer-manipulation problems separate people who memorise from people who understand references. Stacks and queues underpin traversal, parsing, and scheduling questions that show up in both OAs and interviews.

Key concepts

• Fast/slow pointers detect cycles and find the middle of a list.
• Reversing a linked list in place — the classic pointer-rewiring drill.
• Stack (LIFO) for matching brackets, undo, and monotonic-stack problems.
• Queue (FIFO) for BFS and level-order traversal.

Questions that get asked

• Reverse a linked list (iterative and recursive).
• Detect a cycle in a linked list (Floyd's algorithm).
• Merge two sorted linked lists.
• Valid parentheses using a stack.
• Implement a queue using two stacks.