20. Valid Parentheses {Easy}

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 1:

Input: "()"
Output: true
Example 2:

Input: "()[]{}"
Output: true
Example 3:

Input: "(]"
Output: false
Example 4:

Input: "([)]"
Output: false
Example 5:

Input: "{[]}"
Output: true

class Solution:
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """
        left = ['(', '{', '[']
        right = [')', '}', ']']
        stack = []
        for letter in s:
            if letter in left:
                stack.append(letter)
            elif letter in right:
                if len(stack) <= 0:
                    return False
                if left.index(stack.pop()) != right.index(letter):
                    return False
        return len(stack) == 0