TypeScript on LeetCode: From Beginner to Pro in 30 Days

Why TypeScript is the Ideal Language for LeetCode?

If you've been solving LeetCode problems in JavaScript so far, it's time to switch to TypeScript. Why? Because strict typing eliminates a whole class of errors even before running the code. You won't accidentally pass a string where a number is expected, and the compiler itself will tell you which methods are available on an object. It's like driving a car with autopilot — faster, safer, and more enjoyable.

LeetCode is not just a platform for cramming algorithms. It's a training ground where you hone skills that are actually needed in production. And TypeScript today is the de facto standard for enterprise development. Companies like Google, Microsoft, and Airbnb are looking for developers who can write typed code. So by solving LeetCode problems in TypeScript, you kill two birds with one stone: you prepare for interviews and level up real skills.

In this article, we'll break down where to start, which types and data structures to learn, and go through 3 problems of varying difficulty — from easy to hard. Let's go!

What You Need to Know Before Starting

Before diving into the pool of problems, make sure you have a foundation. TypeScript is a superset of JavaScript, so you first need to be confident in JS: closures, promises, array methods, working with objects. Without this, TypeScript will just seem like extra syntactic sugar.

Basic Types That Will Be Useful on LeetCode

• number, string, boolean — the basics, you can't do without them
• number[], string[] — arrays, found in 90% of problems
• Record<string, number> — an object as a dictionary (hash map)
• Map<K, V> — an alternative to an object with order preservation
• Set<T> — for unique values
• Tuple — tuples, useful in problems returning a pair of values
• null | undefined — handling edge cases

Data Structures That Appear Most Often

• Arrays — the absolute foundation. Two-dimensional arrays, sorting, binary search
• Hash tables — for fast lookup (the Two Sum problem is a classic)
• Stack and Queue — for problems on parentheses, BFS, DFS
• Linked Lists — reverse, merge, detect cycle
• Trees — binary trees, binary search trees, traversals
• Graphs — usually in Senior-level interviews

Don't try to learn everything at once. Start with arrays and hash tables — this will cover 70% of Easy and Medium level problems.

How to Set Up the Environment for TypeScript on LeetCode

LeetCode supports TypeScript out of the box. When selecting a language, simply click "TypeScript" in the dropdown. However, if you want to write code locally and test it, here's the minimal setup:

npm init -ynpm install typescript @types/node --save-devnpx tsc --init

Create a file solution.ts and run compilation with the command npx tsc solution.ts && node solution.js. This will help you get used to strict typing and catch errors on the fly.

Breakdown of 3 Problems in TypeScript: From Easy to Hard

Now let's move to practice. I've chosen three problems that perfectly demonstrate the capabilities of TypeScript and are often encountered in interviews.

1. Two Sum (Easy) — A Classic

Problem: Given an array of numbers and a target. Find two numbers whose sum equals the target and return their indices.

function twoSum(nums: number[], target: number): number[] {    const map = new Map<number, number>();

    for (let i = 0; i < nums.length; i++) {        const complement = target - nums[i];        if (map.has(complement)) {            return [map.