Valid Parentheses（stack)

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

分析

loop input, stack 只进头不进尾，遇头压入遇尾弹出计算。

最后判断栈是否空

class Solution:
    def isValid(self, s):
        """
        :type s: str
        :rtype: bool
        """
        stack = []
        left = "({["
        right = ")}]"
        for i in s:
            if i in left:
                stack.append(i)
            else:
                if not stack or stack[-1] != left[right.index(i)]:
                    return False
                else:
                    stack.pop()

        return not stack