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Finish your first coding drill

This guide walks you through completing one coding drill from start to finish, so you understand the full loop: choose a track, read the problem, write a solution, run the hidden tests, and record your progress. It takes about five minutes.

Before you start​

Steps​

  1. Open the tracks hub. Go to Learn → Tracks (/learn/tracks).
  2. Pick a track. Start with JavaScript if you're unsure — it has the most drills.
  3. Open a topic, then a drill. Each card shows difficulty and your progress.
  4. Read the problem and examples. Note the expected input → output pairs above the editor.
  5. Implement the starter code. Replace the // TODO in the provided function signature.
  6. Run. Your code executes locally in a Web Worker and is checked against the drill's tests.
  7. Iterate on failures. Read the actual vs expected output for each failing case and fix your logic.
  8. Use hints if stuck. Each drill has layered hints and a three-tier guide before the full solution.
  9. Reveal the solution. After your tests pass (or after a genuine attempt), open the reference solution, complexity analysis, and follow-ups.
  10. Done. Your completion is saved; your streak and per-track stats update on the dashboard.

Worked example​

A classic first drill is "Two Sum": given an array and a target, return the indices of the two numbers that add up to the target.

function twoSum(nums, target) {
  const seen = new Map();
  for (let i = 0; i < nums.length; i++) {
    const need = target - nums[i];
    if (seen.has(need)) return [seen.get(need), i];
    seen.set(nums[i], i);
  }
  return [-1, -1];
}

Run it and the hidden tests confirm it handles the examples plus edge cases (no match, duplicates). Then read the reference solution and its O(n) complexity note.

Tips​

  • Attempt before revealing. Retrieval beats recognition — the solution unlocks after a real try for a reason.
  • Watch the edge cases. Empty arrays, negatives, and duplicates are where most attempts fail.
  • Keep a streak. One drill a day compounds faster than occasional marathons.

Next​

Try a mock interview to rehearse explaining a solution out loud.