20. Valid Parentheses

URL: LeetCode Problem

Problem Description

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order. Every close bracket has a corresponding open bracket of the same type.

Examples:

• 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 '()[]{}'.

Answer

Intuition

Approach

Complexity

• Time complexity: O(n)

• Space complexity: O(n)

Code

class Solution:
    # Stack
    # O(n) time
    # O(n) space
    def isValid(self, str: str) -> bool:
        dict = {
        ')': '(',
        '}': '{',
        ']': '['
        }
        stack = []
        for s in str:
            if len(stack) > 0 and dict[s] == stack[-1]:
                stack.pop()
            else:
                stack.append(s)
            print(stack)
        if len(stack) == 0:
            return True
        else:
            return False