Python3解leetcode Valid Parentheses

问题描述：

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

解题思路及方法：利用字典进行各个符号之间的配对；利用stack的先进后出特性进行验证

代码：

 class Solution:
     def isValid(self, s: str) -> bool:
         if len(s)%2 != 0: return False
         b = {'(':')', '[':']', '{':'}'}
         stack = []
         for x in s:
             if x in b:
                 stack.append(x)
             else:
                 if stack and x==b[stack.pop()]:
                     continue
                 else :
                     return False
         if stack:
             return False
         else:
             return True