Validate parentheses with one or three bracket types

## Problem Given a string `s` consisting only of bracket characters, determine whether it is **valid**. A string is valid if: - Every opening bracket has a corresponding closing bracket of the same type. - Brackets are closed in the correct order. ### Part A (initial version) `s` contains only `'('` and `')'`. ### Part B (follow-up) `s` may contain three types of brackets: `'()'`, `'[]'`, `'{}'`. ## Input / Output - **Input:** `s` (string) - **Output:** `true` if `s` is valid, otherwise `false` ## Constraints (typical) - `0 <= |s| <= 2 * 10^5` ## Examples - `s = "()"` → `true` - `s = "(()"` → `false` - `s = "([{}])"` → `true` - `s = "([)]"` → `false` ## Discussion prompts - Compare using a **stack** vs using a **counter** (for Part A only). When is a counter sufficient, and why does it fail for Part B?