0020: Valid Parentheses

Problem Statement

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

Example 4:

Input: s = "([])"

Output: true

Example 5:

Input: s = "([)]"

Output: false

Constraints:

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

Code Solution

class Solution:
    def isValid(self, s):        
        stk = []
        for cx, c in enumerate(s):
            if c in '[{(':
                stk.append(c)
                continue
            if c == ')' and stk and stk[-1] == '(':
                stk.pop()
                continue
            if c == ']' and stk and stk[-1] == '[':
                stk.pop()
                continue
            if c == '}' and stk and stk[-1] == '{':
                stk.pop()
                continue
            return False
        return len(stk) == 0