Learning path

Interview Problems

Classic coding interview problems solved step by step: brute force first, then the optimal pattern, in Python.

Start with lesson 1

1

Two Sum

Find two numbers that add up to a target. The most famous interview warm-up, solved in one pass with a hash map.
2 min read
2

Contains Duplicate

Does any value appear twice? Three solutions from slow to fast, and why a set wins.
2 min read
3

Valid Anagram

Are two words made of exactly the same letters? Count letters with a dictionary or Counter.
2 min read
4

Group Anagrams

Put words with the same letters into groups. The trick is choosing a good dictionary key.
2 min read
5

Top K Frequent Elements

Return the k most common numbers. Count with a dictionary, then pick the top k with a heap or bucket sort.
2 min read
6

Product of Array Except Self

For every position, multiply everything except that number, without using division. Prefix and suffix products make it O(n).
2 min read
7

Longest Consecutive Sequence

Find the longest run of consecutive numbers in an unsorted list in O(n), by only counting from the start of each run.
2 min read
8

Valid Palindrome

Does a sentence read the same backwards, ignoring spaces and punctuation? Two pointers walking towards each other.
2 min read
9

Container With Most Water

Pick two walls that hold the most water. Start wide and always move the shorter wall inwards.
2 min read
10

3Sum

Find all unique triplets that sum to zero. Sort, fix one number, then run two pointers on the rest, skipping duplicates.
2 min read
11

Trapping Rain Water

How much rain collects between bars of different heights? The water above each bar depends on the tallest walls on its left and right.
2 min read
12

Best Time to Buy and Sell Stock

Buy once, sell once, maximise profit. Track the cheapest price so far in a single pass.
2 min read
13

Longest Substring Without Repeating Characters

The textbook sliding window problem. Grow the window to the right; when a letter repeats, shrink from the left.
2 min read
14

Minimum Window Substring

Find the smallest part of a string that contains every letter of another string. Expand until valid, then shrink while valid.
2 min read
15

Subarray Sum Equals K

Count continuous subarrays that add up to k, even with negative numbers. Prefix sums plus a hash map of counts.
2 min read
16

Maximum Subarray

Find the continuous part of a list with the biggest sum. Kadane's algorithm: keep going while the running sum helps, restart when it hurts.
2 min read
17

Binary Search

Find a target in a sorted list in O(log n). The bug-free template, first and last position, and search insert position.
2 min read
18

Search in Rotated Sorted Array

A sorted list was rotated at an unknown point. Binary search still works, because one half is always sorted.
2 min read
19

Koko Eating Bananas

Binary search on the answer. Find the slowest eating speed that still finishes all bananas in time.
2 min read
20

Valid Parentheses

Are the brackets balanced and correctly nested? Push opening brackets on a stack; each closing bracket must match the top.
2 min read
21

Min Stack

Design a stack that returns its minimum in O(1). Store the minimum-so-far alongside every item.
2 min read
22

Daily Temperatures

How many days until a warmer day? A monotonic stack of waiting days answers it in one pass.
2 min read
23

Largest Rectangle in Histogram

The biggest rectangle that fits under a bar chart. For each bar, find how far it can stretch left and right with a monotonic stack.
2 min read
24

Reverse Linked List

Flip the direction of every arrow in a linked list with three pointers. Iterative and recursive versions.
2 min read
25

Merge Two Sorted Lists

Zip two sorted linked lists into one sorted list. The dummy node trick removes every special case.
2 min read
26

Linked List Cycle

Does a linked list loop back on itself? A tortoise and a hare running at different speeds will meet if there's a loop.
2 min read
27

LRU Cache

Design a cache that throws out the least recently used item when full, with O(1) get and put. A hash map plus an ordered list.
3 min read
28

Maximum Depth of Binary Tree

How many levels does a tree have? A three-line recursive answer, plus the level-by-level BFS version.
2 min read
29

Invert Binary Tree

Mirror a binary tree by swapping every node's left and right children. Short, famous and a perfect recursion warm-up.
2 min read
30

Binary Tree Level Order Traversal

Return the tree's values level by level. BFS with a queue, processing one whole level per loop.
2 min read
31

Validate Binary Search Tree

Is a tree a valid BST? Checking only parent and child is a trap; pass an allowed range down the tree instead.
2 min read
32

Lowest Common Ancestor of a Binary Tree

Find the deepest node that has both p and q below it. One elegant recursion that reports what it found upwards.
2 min read
33

Kth Largest Element in an Array

Find the k-th largest number without sorting everything. A min-heap of size k, plus quickselect for an average O(n).
2 min read
34

Merge k Sorted Lists

Merge many sorted linked lists into one. A min-heap always tells you which list has the next smallest value.
2 min read
35

Number of Islands

Count groups of connected land in a grid. Each time you step on new land, count it and flood fill the whole island.
2 min read
36

Rotting Oranges

How many minutes until every orange rots? Multi-source BFS where each level of the queue is one minute.
2 min read
37

Course Schedule

Can you finish every course given the prerequisites? Only if there's no cycle. Kahn's algorithm takes courses as soon as they're unlocked.
2 min read
38

Network Delay Time

How long until a signal reaches every computer? Dijkstra's shortest paths, then take the slowest arrival.
2 min read
39

Word Ladder

Change one letter at a time to turn one word into another. Words are nodes, one-letter changes are edges, and BFS finds the shortest ladder.
2 min read
40

Subsets

List every possible subset. For each item, choose "take it" or "skip it"; backtracking explores both.
2 min read
41

Permutations

List every ordering of a set of numbers. Fill positions one by one with numbers not used yet, then undo and try the next.
2 min read
42

Combination Sum

Find every combination of numbers (reusable) that adds up to a target. Backtracking with a start index and early stopping.
2 min read
43

Climbing Stairs

How many ways to climb n stairs taking 1 or 2 steps? The gentlest introduction to dynamic programming.
2 min read
44

House Robber

Take the most money without robbing two neighbouring houses. At each house, choose "rob it" or "skip it", and remember the best so far.
2 min read
45

Coin Change

The fewest coins that make an amount. Why greedy fails, and how a DP table builds every amount from smaller ones.
2 min read
46

Longest Increasing Subsequence

The longest rising sequence you can pick from a list (gaps allowed). An O(n²) DP, then a clever O(n log n) "patience sorting" version.
2 min read
47

Longest Common Subsequence

The longest sequence of letters two strings share in order. The doorway into 2D dynamic programming, with an animated table.
2 min read
48

Edit Distance

The fewest insertions, deletions and replacements to turn one word into another. The algorithm behind spell-checkers and DNA comparison.
2 min read
49

Merge Intervals

Combine overlapping time ranges into one. Sort by start, then extend or start a new block in a single sweep.
2 min read
50

Meeting Rooms II

How many meeting rooms do you need? Sort by start and keep a min-heap of end times, or sweep through start and end events.
2 min read
51

Jump Game

Can you reach the last index if each number is your maximum jump? Track the farthest reachable point in one greedy pass.
2 min read