leetcode20 Valid Parentheses

"""
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 Solution1(object):
    def isValid(self, s):
        stack = []
        match = {'(': ')', '{': '}', '[': ']'}
        for i in s:
            if i == '(' or i == '{' or i == '[':
                stack.append(i)
            else:
                if len(stack) == 0:
                    return False
                top = stack.pop()
                if match[top] != i:
                    return False
        if len(stack) != 0:
            return False
        return True