EasyStack & Queue

Valid Parentheses

Check if parentheses are valid

Solution Approach

Use stack to match opening and closing brackets. Check each closing matches top of stack.

Implementation

def isValid(s):
    stack = []
    pairs = {"(": ")", "[": "]", "{": "}"}
    
    for char in s:
        if char in pairs:
            stack.append(char)
        else:
            if not stack or pairs[stack.pop()] != char:
                return False
    
    return len(stack) == 0

Complexity Analysis

Time Complexity

O(n)

Space Complexity

O(n)

Key Learning Points

LIFO structureMatch pairs efficientlyUseful for bracket problems