intro practice · coding problem
Valid Parentheses
LeetCode #20 — Valid Parentheses. Solve on LeetCode, then implement here if you want it in your drill queue. https://leetcode.com/problems/valid-parentheses/
The mechanism
A stack is last-in-first-out: the last thing you pushed is the first thing you pop. Reach for one when the natural question is "what was the most recent unfinished thing?" — parsing, recursion, undo, matching brackets.
Expected evidence
Pass all LeetCode test cases for this problem.
Try this drill in the practice workspace →
Hints when you need them
- Pattern hint: stack.
- Start with brute force, then optimize time/space.
- Write down invariants before coding.