Blind 75 Stack Questions

1 problem · Free practice

Valid Parentheses is a small problem that tests a useful invariant: the stack contains the opening brackets that still need a match. A closing bracket must pair with the most recent unmatched opening bracket. Counting bracket types alone is insufficient because the order of nesting matters.

Practice stack

Valid Parentheses
Easy
Given a string made of (), [] and {}, decide whether every opener has a matching closer in the correct nested order. The empty string is valid.

What to focus on

Reject a closing bracket when the stack is empty.
Match each closing bracket against the stack top, not any earlier opening bracket.
After the scan, require the stack to be empty.

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