Valid Parentheses

You are given a string s consisting of the following characters: '(', ')', '{', '}', '[' and ']'. The input string s is valid if and only if:

• Every open bracket is closed by the same type of close bracket.
• Open brackets are closed in the correct order.
• Every close bracket has a corresponding open bracket of the same type. Return true if s is a valid string, and false otherwise.