LC 20 - Valid Parentheses

Question

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 '()[]{}'.

Links

Question here and solution here

Solution

concept

Use a stack to keep track the current bracket status. Whenever a matching closing bracket comes in, we can pop the stack.
If it is an opening bracket, we will append to the stack

code

class Solution:
    def isValid(self, s: str) -> bool:
        stack = []
        closeToOpen = { ")" : "(", "]" : "[", "}" : "{" }

        for c in s:
            if c in closeToOpen:
                if stack and stack[-1] == closeToOpen[c]:
                    stack.pop()
                else:
                    return False
            else:
                stack.append(c)
        
        return True if not stack else False

Complexity

time: $O(n)$
space: $O(n)$