Question: You are given a string s consisting of the following characters: '(', ')', '{', '}', '[' and ']'.

The input string s is valid if and only if:

1. Every open bracket is closed by the same type of close bracket.
2. Open brackets are closed in the correct order.
3. Every close bracket has a corresponding open bracket of the same type.

Return true if s is a valid string, and false otherwise.

Example 1:

Input: s = "[]"

Output: true

How to Solve: We have to keep track of the parenthesis as they open, which is perfect for a stack. The reason is the order of the closing matters; if we have 3 openings, [{( , the first closing we should find should be for the most recent parenthesis, ). If we encounter any other first, it is not valid. A stack is a first in last out structure, which is perfect for what we’re looking for.

Answer:

def isValid(s: str) -> bool:
        o_dict = {'(':')', '[':']', '{':'}'}
        current = []
        for v in s:
            if v in o_dict.keys():
                current.append(o_dict[v])
            elif len(current) >= 1 and v == current[-1]:
                current.pop()
            else:
                return False
        return True if not current else False