Stack — Parenthesis

Valid brackets, minimum fixes, scoring or longest valid run.

Practice problems, in order

1. Valid ParenthesesEasy
2. Minimum Add to Make Parentheses ValidMedium
3. Score of ParenthesesMedium
4. Longest Valid ParenthesesHard

How to practise this pattern

1. Read the problem and look for the signal above before writing any code.
2. Say the brute force out loud, with its time complexity, then ask what work it repeats.
3. Apply the key idea and write the optimised version yourself.
4. If you're stuck for more than 10 minutes, take one small hint, not the full solution.
5. Move to the next problem only when you can explain why this pattern fits.