LeetCode Problems

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Not standalone courses — a growing library of problems I explain and solve one at a time. Free learning material: pick a problem, watch the walkthrough, and learn the pattern behind it.

8Problems

3Easy

4Medium

1Hard

8 problems

1Easy

Two Sum

The classic first problem — trade memory for speed with a hash map.

HashingArrayHash Table

Watch walkthrough10 min

20Easy

Valid Parentheses

The textbook case for a stack — match brackets in last-opened, first-closed order.

StackString

Watch walkthrough11 min

121Easy

Best Time to Buy and Sell Stock

One pass, one variable — track the cheapest day you've seen so far.

Greedy / Running MinimumArrayDynamic Programming

Watch walkthrough9 min

3Medium

Longest Substring Without Repeating Characters

A window that grows and shrinks — the canonical sliding-window problem.

Sliding WindowHash TableString

Watch walkthrough15 min

200Medium

Number of Islands

Treat the grid as a graph and 'sink' each island as you find it.

Flood FillArrayDepth-First Search

Watch walkthrough16 min

322Medium

Coin Change

Build the answer for every amount from the ground up — classic DP.

Dynamic ProgrammingArrayBreadth-First Search

Watch walkthrough17 min

56Medium

Merge Intervals

Sort by start, then fold each interval into the last — overlaps collapse.

Sort + SweepArraySorting

Watch walkthrough13 min

42Hard

Trapping Rain Water

The water above each bar depends on the tallest walls on both sides.

Two PointersArrayDynamic Programming

Watch walkthrough20 min