Interview Cracker GitHub

Coding Challenges

Problems grouped by the pattern that solves them, not by difficulty or by data structure. Recognising the pattern is the transferable skill; an individual problem is not.

How to use these

  1. Read the pattern's README.md first — it tells you what signal in a problem statement means "use this pattern".
  2. Open a challenge and read only down to the HINT line.
  3. Give yourself 20 minutes on a blank file. Write the code before running it.
  4. Stuck at 20 minutes? Read the hint, not the solution.
  5. Compare with the solution — including the complexity section.

Every file is runnable and self-testing:

python coding-challenges/two-pointers/valid_palindrome.py

Run the whole suite:

python scripts/run_challenges.py

Patterns

Pattern Use it when Challenges
trie Anything prefix-shaped: autocomplete, "starts with", word search Implement a trie
two-pointers Sorted array; find a pair/triplet; filter in place Valid palindrome, 3Sum
sliding-window Longest/shortest contiguous run satisfying a rule Longest unique substring, max sum of size k
hashing You are searching inside a loop Two sum, group anagrams
binary-search Sorted input, or a monotonic yes/no over a numeric range Search rotated array, minimum eating speed
backtracking You need all arrangements — every subset, permutation or valid board Subsets, combination sum
heap Top-K, Kth largest, a running median, or merging k sorted streams Top K frequent elements
prefix-sum Many range-sum queries, or counting subarrays that sum to k Subarray sum equals k
stack Matching pairs; "next greater/smaller element" Valid parentheses, daily temperatures
linked-list Pointer rewiring; fast/slow traversal Reverse a list, detect a cycle
trees Recursive structure; level-by-level work Level-order traversal, validate a BST
graphs Connectivity, shortest hops, dependency order Number of islands, course schedule
union-find Edges arrive over time; "are these connected yet?"; cycles in an undirected graph Redundant connection
dynamic-programming Overlapping subproblems; count/min/max Climbing stairs, coin change
intervals Ranges that may overlap Merge intervals, meeting rooms

Difficulty

🟢 Easy · 🟡 Medium · 🔴 Hard — the label reflects the insight required, not the amount of code.

A note on these problems

These are long-standing, widely-known problems that appear in textbooks, courses and practice sites. They are here because the underlying pattern is genuinely worth knowing, not because any company is claimed to ask them.

Found a mistake? Edit this page on GitHub.