Foundations: measuring code · Lesson 5 of 23

How to approach any interview problem

A repeatable process from understanding to code to testing, and how to talk while solving.

10 minutes3 games

Most people stumble in interviews not from lack of knowledge but by coding immediately. The interviewer wants to see how you think. Use the same steps every time:

Understand and ask

Examples and edge cases

Brute force

Optimise

Write the code

Test by hand

1. Understand: restate the problem. Ask: can numbers be negative? Duplicates? Empty input? What's the largest size? (Size hints the target: n = 10⁵ means O(n) or O(n log n).) 2. Examples: one normal case and edge cases (empty, one item, all equal). 3. Brute force: state the simple solution and its complexity, even if slow. Now you have something. 4. Optimise: where is work repeated? A hash map? Sorting? Two pointers? 5. Write while talking. 6. Test with your examples, line by line.

If you see…	Think of…
A sorted array	Binary search or two pointers
"Longest/shortest contiguous part"	Sliding window
"Seen this before?" / counting	Hash map / Set
Brackets, "next greater"	Stack
Shortest path, levels	BFS
All combinations	Backtracking
"Number of ways" / "max value" with choices	Dynamic programming
"Top/smallest k"	Heap

You'll see this table come to life through the rest of the track.

Quick question

The problem says n up to 10⁵. Which complexity will usually pass within a second?

Spot the bug

This returns the largest number, but an edge case breaks it. Which line?

Exercise

Fix largest so it works with negative numbers. It must print -2 9.

Expected output: -2 9

JavaScript

function largest(nums) {
  let best = 0
  for (const x of nums) {
    if (x > best) best = x
  }
  return best
}

console.log(largest([-5, -2, -9]), largest([3, 9, 1]))

Practise on real problems

Practice problems that use what you learned here. Solve them in JavaScript, Python or C++.

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