Interview question: Valid Parentheses

Problem Description

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if:

• Open brackets must be closed by the same type of brackets.
• Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example

assert is_valid("()") == True
assert is_valid("()[]{}") == True
assert is_valid("(]") == False