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Top Java Interview Question: First Non-Repeating Character in a String

 

🔹 Introduction

Java interviews often include string-based questions. One of the most commonly asked questions is:

👉 Find the first non-repeating character in a string.

In this article, we will solve it step by step in a simple and easy-to-understand way.

🔹 Problem Statement

Given a string, find the first character that does not repeat.

Example:

Input: aabbcde
Output: c

🔹 Approach

To solve this problem efficiently:

  1. Count frequency of each character
  2. Maintain insertion order
  3. Traverse again to find the first character with frequency = 1

👉 We use LinkedHashMap because:

  • It maintains insertion order
  • Helps us find the first non-repeating character

🔹 Java Code

import java.util.*;

public class FirstNonRepeating {
    public static void main(String[] args) {
        String str = "aabbcde";

        Map<Character, Integer> map = new LinkedHashMap<>();

        // Count frequency
        for (char ch : str.toCharArray()) {
            map.put(ch, map.getOrDefault(ch, 0) + 1);
        }

        // Find first non-repeating
        for (Map.Entry<Character, Integer> entry : map.entrySet()) {
            if (entry.getValue() == 1) {
                System.out.println("First non-repeating character: " + entry.getKey());
                break;
            }
        }
    }
}

🔹 Output

First non-repeating character: c

🔹 Time Complexity

  • O(n) → Efficient solution

🔹 Key Takeaways

✔ Use LinkedHashMap to maintain order
✔ Two-pass solution is optimal
✔ Very commonly asked interview question

🔹 Conclusion

This is a must-know Java interview problem. Practice similar questions to strengthen your problem-solving skills.

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