Finding String Permutations in Java: A Guide to Recursive Methods

🚀 What Will You Learn in This Guide?

In this guide, you will learn step by step how to find all possible permutations of a string in Java.
The recursive approach prevents repetition with the Set structure and
You will see in practice how to produce high-performance results.

🧠 Technical Summary

Topic: Generating String permutations in Java
Purpose: To find all possible sequences (permutations) that can be created from the characters of a string
Solved Problem: Finding different sequences in combination, anagram or sorting algorithms

Basic Steps:

1. Get the first character of the string.
2. Recursively calculate permutations of the remaining characters.
3. Insert the first character into all positions of each subpermutation.
4. Avoid duplication by collecting all results in Set.

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🧩 Java Permutation Algorithm Logic

Example: String = "ABC"

1️⃣ Reserve first character → "A"
2️⃣ Find permutations for the remaining "BC" → "BC", "CB"
3️⃣ Place the character "A" in each position of these results:

BC → ABC, BAC, BCA CB → ACB, CAB, CBA

pgsql

Result: [ABC, ACB, BAC, BCA, CAB, CBA]

🧠 This method generates all combinations recursively.

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💻 Finding String Permutations in Java

import java.util.HashSet;
import java.util.Set;

public class StringPermutasyon {

    // Verilen dizenin tüm permütasyonlarını döndürür.
    public static Set<String> permutationFinder(String str) {
        Set<String> perm = new HashSet<>();

        if (str == null) return null;
        if (str.length() == 0) {
            perm.add("");
            return perm;
        }

        char ilkKarakter = str.charAt(0);
        String kalan = str.substring(1);

        Set<String> altPermutasyonlar = permutationFinder(kalan);

        for (String kelime : altPermutasyonlar) {
            for (int i = 0; i <= kelime.length(); i++) {
                perm.add(charInsert(kelime, ilkKarakter, i));
            }
        }
        return perm;
    }

    // Bir karakteri dizenin belirtilen pozisyonuna ekler.
    public static String charInsert(String str, char c, int j) {
        String bas = str.substring(0, j);
        String son = str.substring(j);
        return bas + c + son;
    }

    // Test için ana metot
    public static void main(String[] args) {
        String d1 = "AAB";
        String d2 = "KLM";

        System.out.println("\nPermütasyonlar " + d1 + " için: \n" + permutationFinder(d1));
        System.out.println("\nPermütasyonlar " + d2 + " için: \n" + permutationFinder(d2));
    }
}

🧠 This code produces unique permutations by filtering repeated letters with the help of Set.

📤 Sample Output

Permütasyonlar AAB için:
[AAB, ABA, BAA]

Permütasyonlar KLM için:
[KLM, KML, LKM, LMK, MKL, MLK]

💡 HashSet automatically eliminates permutations that produce the same result (e.g. repeats in “AAB”).

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🔍 Steps of the Algorithm (PivotTable)

🧩 Step	⚙️ Transaction	💡 Description
1️⃣	First character is taken	For example "JAVA" → "J" + "AVA"
2️⃣	The remaining characters are processed recursively	"AVA" → "AVA", "AAV", "VAA"
3️⃣	The first character is added to each position	"J" → "JAVA", "AJVA", "AVAJ"
4️⃣	Results collected at Set	Duplicate results are automatically eliminated.

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⏱️ Time Complexity

The number of permutations is n! It grows up to (factorial). That is, as the number of characters increases, the processing time increases dramatically.

⚙️ Input Length	🔢 Number of Permutations	⏱️ Time Complexity
3	6	O(n!)
4	24	O(n!)
5	120	O(n!)
6	720	O(n!)

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💬 Frequently Asked Questions (FAQ)

1. Why do we use HashSet?

Because if there are identical characters (like AAB), the Set structure automatically eliminates duplicate results.

2. Why is Set preferred over ArrayList?

While ArrayList stores all results, Set filters out unique values. This is especially important with repetitive characters.

3. Why is the time complexity O(n!)?

Each character is added to different positions, which increases the number of combinations by a factorial ratio.

4. What happens when the string is empty?

The function returns a single empty string ([""]) and does not throw an error.

5. Is there any faster method?

There is no ready-made permutation function in Java. However, for big data, iterative algorithms or parallel processing (multithreading) can be used.

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🏁 Conclusion

In this guide, you learned step by step how to find all permutations of a string in Java. The recursive structure is a perfect example of how to both understand the logic and stand out in interviews. Producing unique results with the use of sets provides a professional approach.

☁️ You can try your own algorithms with performance by testing them in the Java working environment on the GenixNode platform!