20. Valid Parentheses leetcode Python 2016 new Season

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

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

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need O(n) time to go through the whole array and worst case O(n) space to store the array.

class Solution(object):
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """
        stack = []
        for index in range(len(s)):
            current_string = s[index]
            if current_string == '(' or current_string == '[' or current_string == '{':
                stack.append(current_string)
            if current_string == ')':
                if not stack or stack.pop() != '(':
                    return False
            if current_string == ']':
                if not stack or stack.pop() != '[':
                    return False
            if current_string == '}':
                if not stack or stack.pop() != '{':
                    return False
        if stack:
            return False
        return True