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