Equal Quantities
Every opening bracket `(`, `{`, `[` must have a corresponding closing bracket `)`, `}`, `]`. Just counting them isn't enough though. `)(` has equal quantities but is totally invalid!
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Before writing code, you must understand what makes an expression of brackets "valid". It's not just about counting!
Every opening bracket `(`, `{`, `[` must have a corresponding closing bracket `)`, `}`, `]`. Just counting them isn't enough though. `)(` has equal quantities but is totally invalid!
A closing bracket must match the most recently opened unclosed bracket. In `([)]`, the `)` is invalid because the most recently opened bracket was `[`, not `(`.
"Most recently opened" is a perfect description of Last-In-First-Out behavior. This makes a Stack the perfect and only optimal data structure to validate nested bracket patterns.