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This is a easy difficulty Coding & Algorithms question, commonly asked during Onsite rounds at Walmart Labs.

What role is this question designed for?

This question is commonly asked for Machine Learning Engineer candidates at Walmart Labs during technical interviews.

Find first non-repeating character

Last updated: Mar 29, 2026

Quick Overview

This question evaluates string manipulation, frequency-counting concepts, algorithmic complexity reasoning, and attention to edge-case handling in sequence data.

Walmart Labs

Dec 7, 2025, 12:00 AM

Machine Learning Engineer

Onsite

Coding & Algorithms

Given a string s, return the first character that appears exactly once in the string (i.e., the leftmost non-repeating character).

If no such character exists, return an empty string (or a sentinel like "" ).

Examples

Input: "leetcode" → Output: "l"
Input: "aabb" → Output: ""

Constraints

1 ≤ len(s) ≤ 1e5
s contains standard ASCII letters/digits/punctuation (you may assume ASCII for counting).

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