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How to Solve First Letter to Appear Twice Problem

Master the First Letter to Appear Twice LeetCode problem with undetectable real-time assistance. Get instant solutions and explanations during your coding interviews.

PhantomCodeAI generates complete solutions and debugging hints that you can use while explaining your approach, so you stay calm and in control.

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Easy#2351
LeetCode Problem

First Letter to Appear Twice

Given a string s consisting of lowercase English letters, return the first letter to appear twice.

Hash TableStringBit ManipulationCounting

PhantomCodeAI will help you solve this problem in real-time during your interview

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Problem Breakdown

Understanding the First Letter to Appear Twice Problem

Let's break down this LeetCode problem and understand what makes it challenging in interview settings.

Problem Statement

Given a string s consisting of lowercase English letters, return the first letter to appear twice.

EasyProblem #2351
LeetCode

First Letter to Appear Twice

Related Topics

Hash TableStringBit ManipulationCounting

How PhantomCodeAI Helps

Get real-time assistance for First Letter to Appear Twice problems during coding interviews. PhantomCodeAI provides instant solutions and explanations.

Examples

# Example 1

Input
s = "abccbaacz"
Output
"c"

# Example 2

Input
s = "abcdd"
Output
"d"

Constraints

2 <= s.length <= 100
s consists of lowercase English letters.
s has at least one repeated letter.

How PhantomCodeAI Helps with LeetCode Problems

Reinforce undetectability, platform compatibility, and real-time assistance to remove any doubt.

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See PhantomCodeAI in Action

Watch how PhantomCodeAI helps solve LeetCode problems during live interviews

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Solve First Letter to Appear Twice — Given a string s consisting of lowercase English letters, return the first lette...

Here's the optimal approach using Hash Table:

def solve(input):
# Optimal O(n) solution
return result

Time: O(n)  |  Space: O(n)

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Undetectability Checklist

Run compatibility test before interviews
Use recommended Zoom version settings
Enable advanced screen capture options
Verify native desktop architecture
Test with screen recording software

Platform Compatibility

Zoom
Supported
Google Meet
Supported
HackerRank
Supported
CodeSignal
Supported
CoderPad
Supported
Teams
Supported

User results and traction

Thousands of developers use PhantomCodeAI. Social proof signals that this approach helps real candidates land offers across a range of companies.

Undetectability and technical details

Our native desktop architecture avoids common detection vectors used by browser extensions. We provide a clear checklist so you can run basic checks and confirm the app will be invisible.

Platform compatibility and limitations

We work with Zoom, HackerRank, CodeSignal, CoderPad and other web-based platforms. Check the compatibility note and request a browser link if a specific desktop app is unsupported.

Frequently Asked Questions

Common questions about solving First Letter to Appear Twice and using PhantomCodeAI during coding interviews.

How does PhantomCodeAI help with LeetCode problems during a coding interview?
PhantomCodeAI reads the problem from a screenshot or listens to it via audio, then generates a complete solution with a step-by-step approach and time/space complexity, so you can focus on explaining your reasoning rather than getting stuck on implementation.
Which programming languages does PhantomCodeAI support?
11 languages including Python, Java, C++, JavaScript, TypeScript, Go, Rust, Ruby, Swift, Kotlin, and C#. Set a preferred language to get idiomatic solutions.
Does PhantomCodeAI work with HackerRank, CodeSignal, and CoderPad?
Yes — it works alongside LeetCode, HackerRank, CodeSignal, CoderPad, and HackerEarth, plus any browser-based coding environment.
Can PhantomCodeAI handle follow-up questions and modifications?
Yes. Capture the modified problem by screenshot or audio and get an updated solution within seconds, including new edge cases and optimizations.
Is PhantomCodeAI detectable during live interviews?
PhantomCodeAI runs as a native desktop overlay that does not appear in screen shares, recordings, or proctoring software, and works with Zoom, Google Meet, Microsoft Teams, and major coding platforms.
Should I still practice these problems on my own first?
Yes. The most durable preparation is solving problems unaided first, then using AI afterward to review your approach, complexity, and missed edge cases — use it to debrief and learn, not to skip the reps.

Related LeetCode Problems

Practice more problems similar to First Letter to Appear Twice.

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