What is a stack?
Why Interviewers Ask This
This is a classic screening question for Data Structures & Algorithms roles. Hiring managers ask it early in interviews to gauge your baseline understanding and determine if you can communicate technical concepts clearly.
Answer
A stack is a linear data structure following the LIFO (Last In, First Out) principle — the last element inserted is the first one removed, like a stack of plates. Operations: push(item) — add to top: O(1); pop() — remove and return top element: O(1); peek()/top() — view top without removing: O(1); isEmpty() — check if empty: O(1); size(): O(1). Implementation: using an array (with a top pointer) or a linked list (push/pop at head). Applications: (1) Function call stack — programming languages use a call stack to track function invocations, return addresses, and local variables; (2) Undo/redo operations in editors; (3) Browser back button (history stack); (4) Expression evaluation — postfix/infix evaluation; (5) Balanced parentheses validation; (6) DFS traversal (iterative); (7) Backtracking algorithms. Stack overflow: when a recursive function calls itself too many times, exhausting the call stack (recursion depth limit). Use stacks when you need to reverse things, match brackets, or process things in LIFO order.
Common Mistake
A common mistake is memorizing definitions without understanding implications. When asked this question, go one level deeper — explain what happens when this concept is misused or ignored.