Test your understanding with carefully selected problems from easy to hard. Each problem includes detailed solutions, complexity analysis, and step-by-step explanations to help you master stacks and queues.

Practice Objectives

Easy Problems

Build confidence with basics

Medium Problems

Apply advanced techniques

Hard Problems

Master complex scenarios

Coding Skills

Interview preparation

Easy Problems

Start with these fundamental problems to build confidence and understand core concepts. Each problem focuses on basic stack and queue operations.

Valid Parentheses

Easy

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

Examples:

Input: "()"

Output: true

Input: "()[]{}"

Output: true

Input: "(]"

Output: false

Problem Details:

Data Structure:Stack

Time Complexity:O(n)

Space Complexity:O(n)

Approach: Use stack to match opening and closing brackets

Implement Queue using Stacks

Easy

Implement a first in first out (FIFO) queue using only two stacks.

Examples:

Input: push(1), push(2), peek(), pop()

Output: 2, 1

Problem Details:

Data Structure:Stack + Queue

Time Complexity:O(1) amortized

Space Complexity:O(n)

Approach: Use two stacks - one for input, one for output

First Unique Character in a String

Easy

Given a string s, find the first non-repeating character in it and return its index.

Examples:

Input: "leetcode"

Output: 0

Input: "loveleetcode"

Output: 2

Problem Details:

Data Structure:Queue + HashMap

Time Complexity:O(n)

Space Complexity:O(1)

Approach: Use queue to maintain order and map for frequency

Medium Problems

Challenge yourself with these intermediate problems that require deeper understanding of stack and queue properties. Perfect for interview preparation.

Evaluate Reverse Polish Notation

Medium

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Examples:

Input: ["2","1","+","3","*"]

Output: 9

Input: ["4","13","5","/","+"]

Output: 6

Problem Details:

Data Structure:Stack

Time Complexity:O(n)

Space Complexity:O(n)

Approach: Push operands to stack, pop for operators

Daily Temperatures

Medium

Given an array of integers temperatures, return an array where each element is the number of days you have to wait for a warmer temperature.

Examples:

Input: [73,74,75,71,69,72,76,73]

Output: [1,1,4,2,1,1,0,0]

Problem Details:

Data Structure:Stack

Time Complexity:O(n)

Space Complexity:O(n)

Approach: Monotonic stack to find next greater element

Sliding Window Maximum

Medium

Given an array and a sliding window of size k, find the maximum for each window.

Examples:

Input: nums = [1,3,-1,-3,5,3,6,7], k = 3

Output: [3,3,5,5,6,7]

Problem Details:

Data Structure:Deque

Time Complexity:O(n)

Space Complexity:O(k)

Approach: Monotonic deque to maintain maximum

Hard Problems

Master the most challenging stack and queue problems. These require advanced techniques and deep understanding of data structure properties.

Largest Rectangle in Histogram

Hard

Given an array of integers heights representing the histogram's bar height, find the area of the largest rectangle.

Examples:

Input: [2,1,5,6,2,3]

Output: 10

Input: [2,4]

Output: 4

Problem Details:

Data Structure:Stack

Time Complexity:O(n)

Space Complexity:O(n)

Approach: Stack to find previous and next smaller elements

Trapping Rain Water

Hard

Given n non-negative integers representing an elevation map, compute how much water it can trap after raining.

Examples:

Input: [0,1,0,2,1,0,1,3,2,1,2,1]

Output: 6

Input: [4,2,0,3,2,5]

Output: 9

Problem Details:

Data Structure:Stack

Time Complexity:O(n)

Space Complexity:O(n)

Approach: Stack to calculate trapped water level by level

Additional Coding Challenges

Ready for more? Here are additional challenges and resources to further strengthen your stack and queue skills. Perfect for interview preparation!

Pattern Recognition

• Monotonic Stack Problems

• Next Greater/Smaller Element

• Parentheses Matching Variations

• Expression Evaluation Types

• Stack-based Tree Traversals

Focus on recognizing when to use stack vs queue based on problem patterns.

Optimization Techniques

• Space-optimized Solutions

• Amortized Analysis

• Two-pointer with Stack/Queue

• Dynamic Programming + Stack

• Multiple Data Structure Combo

Learn advanced techniques that combine stacks/queues with other algorithms.

Interview Focus

• Time/Space Complexity Analysis

• Edge Case Handling

• Code Optimization

• Alternative Approaches

• Real-world Applications

Practice explaining your solutions and discussing trade-offs clearly.

Recommended Practice Platforms

LeetCode

500+ Stack/Queue problems

★ Recommended for interviews

HackerRank

Data Structures section

★ Good for beginners

CodeForces

Competitive programming

★ Advanced problems

GeeksforGeeks

Comprehensive tutorials

★ Theory + Practice

Suggested Study Plan

Week 1-2: Foundations

Master basic operations and simple problems

Week 3-4: Pattern Recognition

Identify common problem patterns and approaches

Week 5-6: Advanced Problems

Tackle medium and hard difficulty challenges

Week 7-8: Interview Prep

Practice explaining solutions and optimizations

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