Arrays and Hash Map • Beginner • 15 min
Two Sum
How to solve a classic interview problem while explaining the path, the trade-off, and the final choice.
Challenges
Practice problems with walkthroughs, hints, and solutions.
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Arrays and Set • Beginner • 10 min
Contains Duplicate
How to move away from O(n²) by noticing the problem only asks whether you have seen the value before.
Strings and Frequency Count • Beginner • 12 min
Valid Anagram
How to notice that an anagram is about frequency, not order, and justify the move from sorting to counting.
Strings and Two Pointers • Beginner • 12 min
Valid Palindrome
How to notice that the problem asks for mirrored comparison, not a pile of unnecessary preprocessing.
Stack • Beginner • 15 min
Valid Parentheses
How to notice that each closing bracket must match the most recent opening one and why that calls for a stack.
Arrays and Single Pass • Beginner • 15 min
Best Time to Buy and Sell Stock
How to move away from comparing every pair and notice that the problem only needs the lowest price so far.
Arrays and Dynamic Programming • Beginner • 18 min
Maximum Subarray
How to notice that the real question is whether the previous sum still helps or whether it is now hurting.
Search • Beginner • 15 min
Binary Search
How to use sorted order to eliminate half the search space and keep a clear invariant.
Linked List • Beginner • 15 min
Reverse Linked List
How to move away from rebuilding with an array and understand the core pointer idea in a [`linked list`](/glossary/linked-list).
Linked List • Beginner • 18 min
Merge Two Sorted Lists
How to notice the lists are already sorted and that the right comparison always happens at the heads.
Dynamic Programming • Beginner • 15 min
Climbing Stairs
How to notice the recurrence from the last two choices and use that as an entry point to dynamic programming.
Sliding Window • Intermediate • 20 min
Longest Substring Without Repeating Characters
How to notice that the problem asks for a window that expands and contracts, not a full restart at every repetition.
Arrays and Two Pointers • Intermediate • 25 min
3Sum
How to reduce a three-variable problem into [two pointers](/glossary/two-pointers) after sorting and handling duplicates in a controlled way.
Arrays and Two Pointers • Intermediate • 18 min
Container With Most Water
How to justify why only the shorter side makes sense to move and use that to shrink the search.
Hash Map • Intermediate • 18 min
Group Anagrams
How to notice that the real point is not the map itself, but choosing a canonical key that represents the group.
Arrays and Prefix/Suffix • Intermediate • 20 min
Product of Array Except Self
How to use prefix and suffix products to build the local answer for each index from the surrounding context.
Intervals • Intermediate • 20 min
Merge Intervals
How to sort by start time and turn many intervals into one pass that either expands the current block or opens a new one.
Linked List • Intermediate • 15 min
Linked List Cycle
How to detect a cycle with fast and slow pointers and understand why different speeds eventually meet.
Trees • Beginner • 10 min
Invert Binary Tree
How to notice that inverting a tree is just swapping left and right at each node and letting recursion handle the rest.
Trees • Beginner • 10 min
Maximum Depth of Binary Tree
How to measure tree height by thinking about leaves, subtrees, and the combination `1 + max(left, right)`.
Dynamic Programming • Intermediate • 25 min
Coin Change
How to move away from blind combinations and build a DP that searches for the minimum cost for each value up to the target.
Graphs on a Grid • Intermediate • 22 min
Number of Islands
How to see each cell as a graph node and use DFS or BFS to consume one whole island at a time.
Trees • Intermediate • 20 min
Validate Binary Search Tree
How to validate a BST by propagating limits through recursion instead of doing local comparisons that look right but break.
Trees • Intermediate • 18 min
Binary Tree Level Order Traversal
How to traverse a tree level by level with a queue and clearly separate what belongs to the current level before moving down.