#stacks

36 stacks interview questions, asked at Meta, Amazon, Google and 16 more companies. Last asked Oct 2026.

Microorganisms eating neighbours: the largest survivorCodingMicroorganisms stand in a row. Each one has a family (a letter) and a size (a positive integer). A microorganism can eat the neighbour directly to its left or right when that…
Last asked Oct 2026
Maximum frequency stackCodingBuild a stack of integers with push(x) and pop(). pop removes and returns the value that occurs most often in the stack; when several values tie, it takes the one with an…
Last asked Oct 2026
+3
Basic calculator IICodingEvaluate a string expression of non-negative integers with +, -, , / and spaces, no parentheses, respecting precedence (integer division truncates toward zero). Variants reported:…
Last asked Sep 2026
Minimum deletions to balance a parentheses stringCodingA string contains only ( and ). Delete as few characters as possible so the rest is balanced, and return the resulting string.
Last asked Sep 2026
+1
Decode stringCodingA string is encoded with the rule ktext, meaning text repeated k times; k is a positive integer that may have several digits, encoded parts can be nested, and plain letters can…
Last asked Sep 2026
Reconstruct call stacks from enter and exit tracesCodingA profiler gives you call stacks sampled over time, outermost frame first. Turn them into function start and end events, then go the other way: replay start/end events into stacks…
Last asked Sep 2026
+1
Resolve a cd command to an absolute pathCodingWrite a function that takes the current directory and the argument to cd, and returns the directory pwd would print afterwards. Handle ., .., repeated and trailing slashes, and…
Last asked Sep 2026
Is a vehicle ready to go after its maintenance events?CodingA vehicle can leave only when all its maintenance work is closed. Its log is a list of events: each opens a maintenance task of some type or ends the open task of that type…
Last asked Sep 2026
+1
Basic calculatorCodingEvaluate an arithmetic expression string with non-negative integers, +, -, , / (integer division toward zero) and spaces, respecting operator precedence, without eval. State the…
Last asked Aug 2026
+2
Valid parenthesesCodingGiven a string of brackets (), and {}, decide whether every bracket is closed by the matching type in the right order.
Last asked Aug 2026
Nested list weight sum, parsed from a stringCodingA nested list holds integers and other lists. Return the sum of every integer multiplied by its depth, where the outermost list is depth 1. One version hands you the nested list…
Last asked Aug 2026
Queue using two stacksCodingImplement a first-in, first-out queue (push, pop, peek, empty) using only two stacks. Move items from the input stack to the output stack only when the output stack is empty, and…
Last asked Jun 2026
Evaluate a postfix expressionCodingEvaluate an arithmetic expression written in postfix (reverse Polish) notation.
Last asked Jun 2026
Asteroid collisionCodingAsteroids move along a line; each has a size and a direction. When two meet, the smaller one explodes, and both explode if they are the same size. Return the asteroids left after…
Last asked Jun 2026
+2
Exclusive time of functionsCodingFrom function start and end logs, compute each function's exclusive execution time excluding nested calls. Reported on 1point3acres' Decagon bank; reconstructed in our own words.
Last asked May 2026
Command executor with undoCodingImplement a class that executes commands and can undo them: execute(command) applies it and records it, and undo() reverts the most recent command that has not been undone. Decide…
Last asked Apr 2026
Flatten a nested list iteratorCodingA list holds integers and other lists, nested to any depth. Write an iterator with next() and hasnext() that returns the integers in order. Do it lazily with a stack of positions…
Last asked Apr 2026
+1
Largest all-ones square and rectangle in a binary matrixCodingGiven a grid of 0s and 1s, return the area of the largest square made only of 1s. Then extend your approach to the largest all-ones rectangle.
Last asked Apr 2026
+1
Text editor with undo and redoCodingBuild a small text editor supporting insert, delete, undo and redo with correct cursor behavior after each operation. Reported on 1point3acres' Notion bank; reconstructed in our…
Last asked Mar 2026
Construct a BST from its preorder traversalCodingGiven the preorder traversal of a binary search tree with distinct values, rebuild the tree and return its root. Aim for linear time.
Last asked Mar 2026
Final discounted prices and the items sold at full priceCodingEach item in a list gets a discount equal to the price of the first later item that costs no more than it; items with no such later item sell at full price. Return the total paid…
Last asked Mar 2026
Minimum remove to make valid parenthesesCodingA string holds letters and parentheses. Remove the fewest parentheses so that the rest is balanced, and return any valid result. Do it with a stack of indices first, then without…
Last asked Feb 2026
Lazy in-order BST iteratorCodingWrite an iterator over a binary search tree that returns values in sorted order one at a time, with next() and hasnext(). Do it lazily with a stack, so memory is proportional to…
Last asked Feb 2026
Decode a string with nested repeat countsCodingAn encoded string uses k... to mean the bracketed part repeated k times, and brackets may nest, for example 3a2c decodes to accaccacc. Return the decoded string. Use a stack or…
Last asked Feb 2026
Smallest number from an increase and decrease patternCodingGiven a pattern of I (increase) and D (decrease) characters of length n, build the smallest number of n + 1 distinct digits from 1 to 9 whose consecutive digits follow the…
Last asked Feb 2026
Short systems utility tasks: spikes, logs and bracketsCodingSolve a few short utility problems: detect a sudden temperature spike in timestamped CPU readings, check that brackets in an expression balance, and aggregate log lines by status…
Last asked Feb 2026
Remove dictionary words from a character stream as they appearCodingCharacters arrive one at a time. Whenever the characters kept so far end with a word from a given dictionary, remove that word, and keep going (a removal can make a new match…
Last asked Feb 2026
Min stackCodingDesign a stack that supports push, pop, top and returning the current minimum element, each in constant time. Explain how the minimum stays correct after pops, how you handle…
Last asked Feb 2026
Binary search tree iteratorCodingImplement an iterator over a binary search tree that returns the values in ascending order with next() and hasnext(). Each call should take amortised O(1) time, and memory should…
Last asked Jan 2026
Validate a JSON-like stringCodingWrite a validator that decides whether a string is well-formed JSON-like text: balanced braces and brackets, quoted keys, commas and colons in the right places, valid values…
Last asked Jan 2026
Repeatedly remove runs of equal adjacent charactersCodingRemove every run of two or more equal adjacent characters, and keep doing it as new runs form, until none remain. For example xyyyx becomes xx and then the empty string. Do it in…
Last asked Jan 2026
Evaluate an arithmetic expression with variablesCodingEvaluate an arithmetic expression string with numbers, +, -, , /, parentheses and variable names whose values are given in a map. Respect precedence and parentheses, and decide…
Last asked Sep 2025
Remove runs of repeated characters (1D candy crush)CodingRepeatedly remove every run of at least k equal adjacent characters from a string until none is left, and return what remains. Removing a run can join two runs into a new one…
Last asked Sep 2025
Validate a bracket string after digit-driven deletionsCodingA string mixes brackets and digits, for example (())2)1(). Each digit d requires removing exactly d of the brackets that come before it. Decide whether the deletions can be chosen…
Last asked Aug 2025
Parse and evaluate a Lisp-like expression with error handlingCodingEvaluate expressions such as (add 1 2), (mult 3 (add 2 3)) and (let x 2 (mult x 5)), where let binds variables in nested scopes. Unlike the classic version, the input may be…
Last asked Aug 2025
Max stackCodingDesign a stack that also supports peekMax() and popMax() (removing the top-most copy of the largest value), along with push, pop and top. Compare a two-stack design with a doubly…
Last asked Jul 2025