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 · Beginner
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.
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)`.