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，是否是有效的括号串。思路也很简单，能够用栈的方式实现。若是遇到了"[" ,"{" ,"("就入栈。若是遇到了"]" ,"}" ,")" 就出栈前一个字符，看前一个字符是否能匹配当前的字符。若是不行，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，若是是0则表明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