32.leetCode 20. Valid Parentheses

【题目】：

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.

【分析】：

从左到右遍历字符串，将当前字符与前一个的字符进行比较，若成对则将之前的字符删除。若是左括号，则放入到栈中。

【Python代码】：

class Solution:
def isValid(self, s):
"""
:type s: str
:rtype: bool
"""
if len(s)%2 == 1:
return False
map = {')':'(',']':'[','}':'{'}
brackets = []

for item in s:
if brackets:
bracket = brackets[-1]
else:
brackets.append(item)
continue

if item in map.values():
brackets.append(item)
if item in map.keys():
if map[item] == bracket:
brackets.pop()
else:
return False

if brackets:
return False
else:
return True