EasyStack & Queue

Implement Queue using Stacks

Queue using two stacks

Solution Approach

Use Stack for LIFO problems. Useful for matching pairs (parentheses), expression evaluation, or monotonic patterns. Maintain stack of elements and pop when condition met.

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