20. Valid Parentheses

20. Valid Parentheses

1. 题目

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

2. 思路

本题是验证给定的字符串，是不是有效的括号串。思路也很简单，可以用栈的方式实现。如果遇到了"[" ,"{" ,"("就入栈。如果遇到了"]" ,"}" ,")" 就出栈前一个字符，看前一个字符是否能匹配当前的字符。如果不行，则当前字符也入栈。最后程序结束的时候判断栈里面还有多少元素，如果是0则代表字符串是有效的括号串。

3. 实现

class Solution(object):
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """
        stack = []
        if len(s) == 0 : return True

        for e in s :
            if len(stack) == 0 :
                stack.append(e)
                continue
            t = stack[-1]
            if ( t == "(" and e == ")" ) or ( t == "{" and e == "}" ) or ( t == "[" and e == "]" ):
                stack.pop(-1)
            else:
                stack.append(e)
        if len(stack) == 0 :return True
        return False