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easyDSA

How do you check for valid/balanced parentheses?

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01

Understand the problem

Explain the stack approach.

stackparentheses
02

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03

Study the solution

Push opening brackets onto a stack; on a closing bracket, pop and verify it matches. The string is valid if every close matches and the stack is empty at the end. O(n) time and space.

Solution ready — 2 min read

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04

Read the code

Stack-based validator
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def is_valid(s):
    pairs = {')': '(', ']': '[', '}': '{'}
    stack = []
    for c in s:
        if c in '([{':
            stack.append(c)
        elif not stack or stack.pop() != pairs[c]:
            return False
    return not stack


# --- demo ---
print(is_valid("()[]{}"))   # True
print(is_valid("(]"))       # False
05

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