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20. Valid Parentheses

Difficulty Topics

Description

Given a string s 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.
  3. Every close bracket has a corresponding open bracket of the same type.

 

Example 1:

Input: s = "()"
Output: true

Example 2:

Input: s = "()[]{}"
Output: true

Example 3:

Input: s = "(]"
Output: false

 

Constraints:

  • 1 <= s.length <= 104
  • s consists of parentheses only '()[]{}'.

Solution

valid-parentheses.py
class Solution:
    def isValid(self, s: str) -> bool:
        mp = {")" : "(", "}" : "{", "]" : "["}
        stack = []

        for x in s:
            if x in mp:
                if not stack or stack[-1] != mp[x]:
                    return False
                stack.pop()
            else:
                stack.append(x)

        return len(stack) == 0