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Exam-Pattern Mock TestsBasic & Advanced AssessmentsTimed Test EnvironmentCertification-Ready Practice14,200+ Learners Preparing NowTrusted by Professionals Across IndustriesExam-Pattern Mock TestsBasic & Advanced AssessmentsTimed Test EnvironmentCertification-Ready Practice14,200+ Learners Preparing NowTrusted by Professionals Across Industries

SkillSkill isn'tisn't whatwhat youyou know,know, it'sit's whatwhat youyou cancan prove.prove.AA lotlot ofof peoplepeople prepare,prepare, butbut stillstill feelfeel unsureunsure whenwhen it'sit's timetime toto perform.perform. NotNot becausebecause theythey lacklack knowledgeknowledge butbut becausebecause they'vethey've nevernever testedtested itit underunder realreal conditions.conditions.That'sThat's thethe gapgap mostmost platformsplatforms ignore.ignore.OnSkillOnSkill isis builtbuilt toto closeclose itit turningturning practicepractice intointo somethingsomething youyou cancan measure,measure, improve,improve, andand relyrely onon whenwhen itit actuallyactually matters.matters.

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MediumData Structures

Longest Valid Parentheses

Given a string s containing just ( and ), return the length of the longest valid (well-formed) parentheses substring.

A substring is valid if every opening parenthesis has a corresponding closing parenthesis in the correct order.

Input: s = "(()"
Output: 2
Explanation: The longest valid substring is "()".

Input: s = ")()())"
Output: 4
Explanation: The longest valid substring is "()()".

Python 3▾

Auto Save

def longestValidParentheses(s: str) -> int:
    stack = [-1]
    max_len = 0
    for i, char in enumerate(s):
        if char == '(':
            stack.append(i)
        else:
            stack.pop()
            if not stack:
                stack.append(i)
            else:
                max_len = max(max_len, i - stack[-1])
    return max_len

Test Case 1

✓

Input: s="(()"
Expected: 2
Output: 2

Test Case 2

✓

Input: s=")()())"
Expected: 4
Output: 4

Test Case 3

✓

Input: s="()(())"
Expected: 6
Output: 6

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Your last five full-length exam attempts

Your ScoreAverage

Readiness Score

82%+6%

Accuracy

76%+4%

Your Rank

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Avg Time/Q

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Current Strengths

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Priority Focus Areas

Your accuracy in Operating Systems is currently at 38%. We recommend reviewing concurrency and memory management before attempting another full mock test.

Everything you need to know.

What exactly is OnSkill?

OnSkill is a performance-focused certification exam preparation platform that helps you practice through real exam-pattern mock tests and assessments. Instead of just learning concepts, it helps you understand how well you can apply your knowledge in actual exam conditions.

Is OnSkill suitable for both beginners and experienced learners?

Yes, OnSkill is designed for all levels. If you're a beginner, you can build your fundamentals step by step. If you already have knowledge, you can use it as a mock test and practice platform to test your readiness and improve your performance.

What are 4 features in OnSkill, and how do they help in my preparation?

OnSkill is built around four key parts, each supporting a different stage of your exam preparation journey. Walkways gives you a clear roadmap for certification preparation. Base Ground helps you build fundamentals through structured practice. Proving Ground allows you to take full-length mock tests in a real exam environment. And Xone helps you practice interview questions for technical interview preparation.

What does Walkways do in OnSkill?

Walkways helps you plan your certification exam preparation by providing a clear roadmap based on your goal and current level, so you always know what to focus on next.

What is Base Ground used for?

Base Ground helps you build strong fundamentals for certification exam preparation through structured practice before moving on to advanced mock tests.

What is Proving Ground in OnSkill?

Proving Ground is where you take full-length online mock tests in a real exam-like environment to test your performance under actual exam conditions.

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Snowflake SnowPro Advanced Architect

AWS Machine Learning - Specialty

AWS Solution Architect Professional

AWS Solutions Architect Associate

Databricks Certified Generative AI Engineer Associate

50+ Certifications in One Preparation Platform

The AI Interview Practice Built Around Real Hiring Standards.

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Your certification prep, all in one screen.

Base Ground

Performance Trend

Current Strengths

Priority Focus Areas

Everything you need to know.

What exactly is OnSkill?

Is OnSkill suitable for both beginners and experienced learners?

What are 4 features in OnSkill, and how do they help in my preparation?

What does Walkways do in OnSkill?

What is Base Ground used for?

What is Proving Ground in OnSkill?

First Mock Test.First Reality Check.

Start From Scratch.
End Up Certified.

Stop Figuring It Out Alone.
Your AI Career Guide Is Here.

First Mock Test.
First Reality Check.