Coding Interview Preparation on Telegram: “🎯 CODING CHALLENGE #2

🎯 CODING CHALLENGE #2 - Valid Parentheses
Difficulty: Easy | Asked at: Microsoft, Meta, Bloomberg

Given a string containing just (, ), {, }, [, ], determine if the input is valid. Brackets must close in the correct order.


Input: "{[()]}"  → true
Input: "{[(])}"  → false
Input: "((("     → false

💡 Hint: What data structure naturally handles "last opened, first closed"?

Solution:

python
def is_valid(s):
    stack = []
    pairs = {')': '(', ']': '[', '}': '{'}
    for char in s:
        if char in pairs.values():
            stack.append(char)
        elif char in pairs:
            if not stack or stack.pop() != pairs[char]:
                return False
        else:
            return False
    return not stack

Complexity: O(n) time, O(n) space (worst case, all opening brackets).

Common mistake: Forgetting to check if the stack is empty at the very end. "(((" never fails inside the loop - you only catch it because the stack still has unclosed brackets when you finish.

Stacks show up constantly in interviews. Where else have you seen one used? 👇