Valid Parentheses — Day 7(Python)

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Today we will be looking into one of Leetcode’s Easy tagged questions. Let us jump into the problem without losing much time.

20. Valid Parentheses

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.

Example 1:

Input: s = "()"
Output: true

Example 2:

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

Example 3:

Input: s = "(]"
Output: false

Example 4:

Input: s = "([)]"
Output: false

Example 5:

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


  • 1 <= s.length <= 104


Since we have a limited set of brackets to worry about in this problem, we can use a dictionary that holds the key as closing brackets and value as a respective opening bracket. We need a stack which will hold the order of opening brackets, we can remove the opening brackets from the stack when we find the right closing bracket.

  1. Construct a dictionary that holds opening brackets (value) and closing brackets(key).
class Solution:
def isValid(self, s: str) -> bool:
dictionary_brackets = {'}':'{',')':'(', ']':'['}
open_bracket = ('{','(','[')
stack_bracket = []
for each_char in s:
if stack_bracket == [] or each_char in open_bracket:
return False
return False if stack_bracket else True

Complexity analysis

Time Complexity

We would be required to traverse through each character in the string which would take O(N).

Space Complexity

In worst-case-scenario, if our input string is ‘(((([[[[[{{{{’. The entire string will be stored in the stack, which means the space complexity will be O(N).

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