Valid Parentheses
s holds only bracket characters: ( ) [ ] { } Return true when every opener is closed by its matching closer, innermost first Example: "[{}]()" => true, "{(})" => false, "((" => false
This is a stack/invariant problem: once your stack represents the right thing, the rest is routine. It's a great warm-up that builds stack invariants without a ton of moving parts. Keep the stack invariant explicit; if it breaks, everything downstream breaks too.
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Each locked part opens in the editor once the part before it passes.