advanced
13 min read
N-Queens
The board is only the output surface. The real N-Queens solution is a depth-n search over column assignments, with three constraints checked before each…
View solutionCoding interview problem solving
Derive interview solutions instead of memorizing tricks.
Practice familiar coding problems by making the reasoning visible: structural clues, brute-force baselines, invariants, optimization steps, complexity, edge cases, dry runs, and clean implementation.
Our approach
Learn how to derive coding-interview solutions, not just recognize them afterward
AlgoGrinder starts from familiar coding-interview patterns but spends its effort on the missing middle: how you notice the pattern, what clues matter, how you split the problem into subproblems, which invariant makes the solution work, and how the reasoning turns into code. Python is the primary implementation language at launch, with the reasoning kept portable to JavaScript and Java.
Editorial standards
AlgoGrinder Team
Algorithms editorial team
Coding interview problem explanations focused on pattern recognition, invariants, complexity, edge cases, and reliable implementation.
- Cover one named coding problem per problem article and place it under its primary solution pattern.
- Show how to recognize the pattern from constraints and structural clues before presenting the optimized solution.
- Break the problem into explicit subproblems or obligations and show how each part of the algorithm satisfies one of them.
Problem patterns
Browse coding interview patterns
Choose a solution pattern and practice recognizing the structural signals that point toward it before reading the final algorithm.
foundation
3 articles
Arrays & Hashing
Use arrays, hash maps, and sets for fast lookup, counting, grouping, and deduplication.
Explore patternscore-pattern
10 articles
Two Pointers
Coordinate two indices to shrink the search space in arrays, strings, and partitions.
Explore patternscore-pattern
3 articles
Sliding Window
Maintain a moving range to solve contiguous subarray and substring problems efficiently.
Explore patternscore-pattern
1 article
Intervals
Reason about overlap, containment, gaps, scheduling, and interval boundaries.
Explore patternscore-pattern
1 article
Merge Intervals
Sort and combine overlapping intervals into the smallest set of disjoint ranges.
Explore patternscore-pattern
9 articles
In-Place Linked List Manipulation
Reverse, splice, partition, rotate, and delete linked-list nodes safely in place.
Explore patternsintermediate
1 article
K-Way Merge
Merge multiple sorted sources by tracking the next candidate from each one.
Explore patternscore-pattern
6 articles
Modified Binary Search
Use binary search for ordered lookup, boundaries, rotated arrays, peaks, and monotonic partitions.
Explore patternsintermediate
1 article
Binary Search on the Answer
Binary-search a numeric answer space when feasibility changes monotonically.
Explore patternscore-pattern
1 article
Subsets & Combinatorial Generation
Generate subsets and related combinatorial families with clean include-or-exclude reasoning.
Explore patternsintermediate
3 articles
Greedy Techniques
Make provably safe local choices that lead to a globally optimal result.
Explore patternsintermediate
13 articles
Backtracking
Explore constrained choices recursively, prune invalid paths, and undo state cleanly.
Explore patternsintermediate
5 articles
One-Dimensional Dynamic Programming
Solve problems with one-dimensional DP states, recurrences, base cases, and memory compression.
Explore patternsadvanced
9 articles
Two-Dimensional Dynamic Programming
Model DP problems that naturally require two coordinates or progress dimensions.
Explore patternscore-pattern
1 article
Cyclic Sort & Index Placement
Place values at natural indices to expose missing, duplicate, or displaced values.
Explore patternscore-pattern
4 articles
Matrices & Grid Transformations
Manipulate 2D grids with rotations, traversals, markings, layers, and in-place transformations.
Explore patternscore-pattern
2 articles
Stacks
Use LIFO state for nesting, parsing, simulation, path simplification, and deferred work.
Explore patternsintermediate
2 articles
Monotonic Stacks & Queues
Maintain monotonic candidates for next-greater, histogram, and sliding-window problems.
Explore patternscore-pattern
4 articles
Tree Depth-First Search
Use DFS to propagate subtree results, path state, and structural information through trees.
Explore patternscore-pattern
18 articles
State Tracking & Invariants
Track the smallest evolving state needed to preserve a scan or simulation invariant.
Explore patternsintermediate
2 articles
Bitwise Manipulation
Use XOR, masks, shifts, and binary representation as the core problem-solving model.
Explore patternsadvanced
1 article
Divide & Conquer
Split a problem into independent subproblems and combine their results recursively.
Explore patternsFeatured problems
Practice high-value interview patterns
Start with problems that teach transferable reasoning patterns rather than one-off tricks.
intermediate
8 min read
Valid Sudoku
A Sudoku validator does not solve the puzzle. It tracks whether the digits already placed violate any row, column, or 3×3 box constraint.
View solution
intermediate
14 min read
Word Search
A grid DFS can match the right letters and still be wrong. The missing piece is path-local state: mark a cell when you enter it, explore from that choice,…
View solutionLatest
Recently added problem solutions
New coding interview problems with explicit derivation, complexity analysis, edge cases, and implementation reasoning.
intermediate
12 min read
3Sum Closest
The target does not identify the winning triplet. It tells each pointer which direction is still worth exploring.
View solution
intermediate
10 min read
3Sum
A reliable 3Sum solution comes from turning a cubic search into a sequence of sorted two-sum scans—and proving why each pointer move is safe.
View solution
advanced
13 min read
4Sum
Four choices suggest an O(n^4) search. Sorting changes the last two choices into a controlled walk.
View solution
beginner
11 min read
Add Binary
You receive two binary strings, a and b, and must return their sum as another binary string. The inputs contain only '0' and '1', have lengths from 1 to…
View solution
intermediate
10 min read
Add Two Numbers
The lists already expose digits in the order addition needs. Scan both lists together, track one carry, and keep going until there is no digit or carry…
View solution
beginner
10 min read
Binary Tree Inorder Traversal
The difficult part of inorder traversal is not remembering “left, node, right.” It is preserving the parent node while the left subtree is still…
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