Q: How do you reverse a linked list?

**Intuition.** Walk the list once, flipping each next pointer to point **backwards**. You carry three references — prev , curr , next — so you never lose the rest of the list when you rewire a node. <svg viewBox="0 0 520 170" style="width:100%;display:block" xmlns="http://www.w3.org/2000/svg"

Q: How do you solve the climbing stairs problem?

**Intuition.** To reach step n you arrived either from step n-1 (a 1-step) or step n-2 (a 2-step). So the number of ways to reach n is the sum of the ways to reach those two — which is exactly the **Fibonacci** recurrence. <svg viewBox="0 0 520 170" style="width:100%;display:block" xmlns="ht

Q: How do you find the missing number in 0..n?

**Intuition.** The set 0..n has a known total ( n(n+1)/2 ). Subtract the actual array sum and the difference is the missing number. An XOR variant achieves the same with no overflow risk. <svg viewBox="0 0 520 120" style="width:100%;display:block" xmlns="http://www.w3.org/2000/svg" font-family="ui-sans-serif,system-ui"

Question 1

How do you reverse a linked list?

Accepted Answer

**Intuition.** Walk the list once, flipping each next pointer to point **backwards**. You carry three references — prev, curr, next — so you never lose the rest of the list when you rewire a node.

Question 2

How do you solve the climbing stairs problem?

Accepted Answer

**Intuition.** To reach step n you arrived either from step n-1 (a 1-step) or step n-2 (a 2-step). So the number of ways to reach n is the sum of the ways to reach those two — which is exactly the **Fibonacci** recurrence.

Question 3

What is the difference between an array and a linked list?

Accepted Answer

**Intuition.** Both store a sequence, but they make opposite trade-offs. An **array** is one contiguous block — instant random access, painful inserts. A **linked list** is scattered nodes joined by pointers — cheap inserts, no random access.

Question 4

What is the difference between a stack and a queue?

Accepted Answer

**Intuition.** Both are linear collections; they differ only in *which end you take from*. A **stack** is LIFO — last in, first out, like a pile of plates. A **queue** is FIFO — first in, first out, like a line at a counter.

Question 5

What is Big-O notation?

Accepted Answer

**Intuition.** Big-O answers one question: *as the input grows, how fast does the work grow?* It throws away constants and hardware speed and keeps only the dominant term, so you can compare algorithms on a whiteboard instead of a benchmark.

Question 6

How do you find the intersection node of two linked lists?

Accepted Answer

**Intuition.** Two lists that intersect share a common tail. The elegant trick: walk pointer a through list A then list B, and pointer b through list B then list A. Both travel lenA + lenB steps total, so they **align at the intersection** (or both reach null together if there's none).

Question 7

How do you find the missing number in 0..n?

Accepted Answer

**Intuition.** The set 0..n has a known total (n(n+1)/2). Subtract the actual array sum and the difference is the missing number. An XOR variant achieves the same with no overflow risk.

Question 8

How do you find the best time to buy and sell a stock?

Accepted Answer

**Intuition.** You want the biggest "sell minus buy" where the sell day comes *after* the buy day. Sweep once, tracking the **lowest price seen so far** — at each day the best profit if you sold today is price − minSoFar. Keep the maximum of those.

Question 9

How do you merge two sorted linked lists?

Accepted Answer

**Intuition.** Like the merge step of merge sort, but on linked nodes. Use a **dummy head** so you don't special-case the first node; a tail pointer always attaches the smaller of the two current nodes, then advances.

Question 10

How do you add one to a number represented as a digit array?

Accepted Answer

**Intuition.** The number is stored most-significant digit first. Adding one only affects the end — unless you hit a chain of 9s. Walk from the **last digit**: if it's less than 9, increment and you're done; if it's 9, set it to 0 and carry left. If the carry runs off the front (all 9s), prepend a 1.

Data Structures & Algorithms Interview Library

Question Catalog

How do you reverse a linked list?

How do you solve the climbing stairs problem?

What is the difference between an array and a linked list?

What is the difference between a stack and a queue?

What is Big-O notation?

How do you find the intersection node of two linked lists?

How do you find the missing number in 0..n?

How do you find the best time to buy and sell a stock?

How do you merge two sorted linked lists?

How do you add one to a number represented as a digit array?

What is the time complexity of common operations on arrays, hash maps, and balanced trees?

How do you move all zeroes to the end of an array in place?

How do you check for valid/balanced parentheses?

How do you compute the GCD of two numbers?

How do you check if a root-to-leaf path sums to a target?

What is a stable sorting algorithm?

How do you check if two strings are anagrams?

How do you check if a binary tree is symmetric?

How do you find the single number where every other element appears twice?

How do you find the majority element?

How do you check if a linked list is a palindrome?

How do you find a duplicate in an array?

How do you find the maximum depth of a binary tree?

How do you solve the Two Sum problem efficiently?

What is a trie and when is it useful?

How do you find the lowest common ancestor in a binary tree?

How do you find the k closest points to the origin?

How do you solve the 0/1 knapsack problem?

How does binary search work and what are its requirements?

How do you solve the course schedule (prerequisites) problem?

How do you add two numbers stored as linked lists?

How does Quickselect find the kth smallest/largest element?

How do you implement a queue using two stacks?

How do you check if a binary tree is balanced?

How do you check if a graph is bipartite?

How do you remove the nth node from the end of a linked list?

How do you search in a rotated sorted array?

How do you rotate an array by k positions?

How does a hash table work?

How do you validate that a binary tree is a BST?

How do you sort an array of 0s, 1s, and 2s?

How do you find the maximum subarray sum?

Find the kth largest element in an array

How do you get the right-side view of a binary tree?

What is a monotonic stack and when do you use it?

How do you find the middle of a linked list?

How do you compute the diameter of a binary tree?

How do you count the number of set bits in an integer?

How do you count connected components in an undirected graph?

What is a heap / priority queue?

How do you rotate an N×N matrix by 90 degrees in place?

How do you find the kth smallest element in a BST?

What is recursion and what is a base case?

What is a binary search tree (BST)?

How do you do a level-order (BFS) traversal of a tree?

How do you count unique paths in a grid?

What is binary search on the answer?

How do you find the node where a linked list cycle begins?

How do you merge overlapping intervals?

How do you group anagrams together?

How do you find the longest substring without repeating characters?

What is the difference between BFS and DFS?

How do you find the maximum product subarray?

How do you solve the house robber problem?

How do you solve the word break problem?

How do you detect a cycle in a linked list?

How do you traverse a matrix in spiral order?

How do you compute the product of an array except self without division?

How do you solve the container with most water problem?

How do you find the minimum number of meeting rooms required?

How do you solve the coin change problem?

How do you find the longest common subsequence?

How do you find a peak element in an array?

How do you count subarrays that sum to k?

How do you solve the 3Sum problem?

How do you merge two sorted arrays?

How do you find the longest increasing subsequence?

How do you clone a graph?