Data Structures & Algorithms

Number of Islands: Exploring a Grid with Depth-First Search

Count connected regions of land with iterative depth-first search, track visited cells, and implement Number of Islands in C#, Go, and Python 3.

Reverse Integer: Reversing Digits Without Overflow

Reverse a signed integer one digit at a time, detect 32-bit overflow before it happens, and handle negative numbers in C#, Go, and Python 3.

Valid Parentheses: Matching Brackets with a Stack

Learn why bracket order needs a stack, trace a linear-time solution, and implement Valid Parentheses in C#, Go, and Python 3.

Binary Search

Find a target in a sorted array with binary search, using an inclusive search range and O(log n) time.

Two Sum: From Brute Force to a Hash Map

Improve Two Sum from nested loops to a single pass using a dictionary, with C#, Go, and Python 3 examples.

Premium

Advanced Sorting: Working Within a Memory Budget

Choose a sorting strategy around memory limits, stable ordering, and input size, then combine sorted runs with a streaming Python merge.

Basic DSA Patterns: Building Blocks for Problem Solving

Learn reusable DSA patterns for swapping values, using binary search, and finding the first and last occurrence of a target.

Merge Sort — Top Down and Bottom Up for Arrays and Linked Lists

Learn merge sort from first principles: merge sorted ranges, compare top-down and bottom-up arrays, implement linked-list sorting, and analyze stability, time, and space.

Sorting Algorithms: Quick Sort — Practical Deep Dive

Learn how quick sort partitions arrays, follow a step-by-step example, compare recursive and iterative C# implementations, and explore versions in Python and Go.