Data structures and algorithms are the part of coding interviews most people dread. A lot of tutorials start with code. We wanted to start with something you already understand.
Every data structure is something you've seen before
- Stack → a pile of plates. You add to the top and take from the top. Last in, first out. That's your browser's Back button and every function call in your program.
- Queue → the line at a coffee shop. First come, first served.
- Array → a row of numbered lockers. Know the number, walk straight to it.
- Hash map → a coat check. The ticket tells you exactly which hook your coat is on.
- Linked list → a train. Each car only knows the car behind it.
- Tree → a family tree, drawn upside down with the root at the top.
- Graph → a subway map. Stations are nodes, tracks are edges.
- Binary search → finding a name in a phone book. Open the middle, throw away half.
- Dynamic programming → writing each answer down once so you never work it out twice.
Once the picture clicks, the code is mostly bookkeeping. Here's an example.
Example: the sliding window
Problem: you have your step counts for each (in thousands). What's the shortest run of
consecutive days with a total of at least 7?
The brute-force way checks every possible run: O(n²). The picture that helps is a window sliding along the days:
- Grow the window to the right until the total reaches 7.
[2, 3, 1, 2]= 8 → length 4 - Shrink it from the left while the total is still at least 7, recording the length each time.
[1, 2, 4]= 7 → length 3 - Keep going to the end.
[4, 3]= 7 → length 2
Each day enters the window once and leaves it o
def shortest_stretch(days, target): best = float("inf") total = left = 0 for right, steps in enumerate(days): total += steps # grow while total >= target: # sugh best = min(best, right - left + 1) total -= days[left] left += 1 return 0 if best == float("inf") else best print(shortest_stretch([2, 3, 1, 2, 4, 3], 7)) function shortestStretch(days, target) { let best = Infinity, total = 0, left = 0; for (let right = 0; right < days.length; righ total += days[right]; // grow while (total >= target) { // ough best = Math.min(best, right - left + 1); total -= days[left++]; } } return best === Infinity ? 0 : best; } console.log(shortestStretch([2, 3, 1, 2, 4, 3], The pattern to remember: grow from the right, shrink from the left, and record the answer while the window is valid.
The same shape solves "longest substring withoud a lot of other window problems.
Try it yourself
We put this approach into a free practice secti
- 8 short lessons, from arrays and hashing ch with a picture, diagrams you step through with Back/Next, and a quick quiz
- 24 original interview problems (3 per tope in your browser in Python or JavaScript, with tests that show exactly which cases pass. Your code runs privately in your own browser.
- Hints one step at a time when you're stucsolution with time and space complexity
There's also a weekly leaderboard if you liign in (free) to earn 10/20/30 points for
easy/medium/hard problems. It resets every Monday, and each week's top learners get a free, verifiable certificate.
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