[LeetCode][python3]0020. Valid Parentheses

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N2I -2020.04.02

1. My first solution

class Solution:
    def isValid(self, s: str) -> bool:
        upper=['s']
        for item in s:
            if item in ['(','[','{']:
                upper.append(item)
            if item==')':
                if upper.pop(-1)!='(':
                    return False
            if item==']':
                if upper.pop(-1)!='[':
                    return False
            if item=='}':
                if upper.pop(-1)!='{':
                    return False
        if upper==['s']:
            return True
        return False

Explanation:

The solution use a buffer to solve this problem. For the elements ['(','[','{'], has to be matched in the sequence “Last in, First out”. And the element ['s'] to check if buffer is empty.

2. A clean solution

class Solution:
    def isValid(self, s: str) -> bool:
        mapping = {')':'(', '}':'{', ']':'['}        
        stack = []        
        if not s:
            return True       
        for ele in s:
            if ele in mapping:
                if stack:
                    top = stack.pop() 
                else:
                    top = '$'
                if top != mapping[ele]:
                    return False
            else:
                stack.append(ele)        
        return not stack

Explanation:

A more flexible way to code the script.