Valid Parentheses

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

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. 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

My solution:

class Solution:
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """
        if not s:
            return True

        elif len(s) % 2 == 1:
            return False

        stack = list()
        for i in range(len(s)):
            if s[i] == '(':
                stack.append(s[i])
            elif s[i] == '[':
                stack.append(s[i])
            elif s[i] == '{':
                stack.append(s[i])
            elif s[i] == ')':
                if len(stack) <= 0 or stack.pop() != '(':
                    return False
            elif s[i] == ']':
                if len(stack) <= 0 or stack.pop() != '[':
                    return False
            elif s[i] == '}':
                if len(stack) <= 0 or stack.pop() != '{':
                    return False

        if len(stack) > 0:
            return False

        return True