Java/src/main/java/com/thealgorithms/strings/PermuteString.java

package com.thealgorithms.strings;

/*
Backtracking algorithm used in the program:-

>>Fix a character in the first position and swap the rest of the character with the first character.
  Like in ABC, in the first iteration three strings are formed: ABC, BAC, and CBA by swapping A with
  A, B and C respectively.
>>Repeat step 1 for the rest of the characters like fixing second character B and so on.
>>Now swap again to go back to the previous position. E.g., from ABC, we formed ABC by fixing B
again, and we backtrack to the previous position and swap B with C. So, now we got ABC and ACB.
>>Repeat these steps for BAC and CBA, to get all the permutations.
 */
public final class PermuteString {
    private PermuteString() {
    }

    // Function for swapping the characters at position I with character at position j
    public static String swapString(String a, int i, int j) {
        char[] b = a.toCharArray();
        char ch;
        ch = b[i];
        b[i] = b[j];
        b[j] = ch;
        return String.valueOf(b);
    }

    public static void main(String[] args) {
        String str = "ABC";
        int len = str.length();
        System.out.println("All the permutations of the string are: ");
        generatePermutation(str, 0, len);
    }

    // Function for generating different permutations of the string
    public static void generatePermutation(String str, int start, int end) {
        // Prints the permutations
        if (start == end - 1) {
            System.out.println(str);
        } else {
            for (int i = start; i < end; i++) {
                // Swapping the string by fixing a character
                str = swapString(str, start, i);
                // Recursively calling function generatePermutation() for rest of the characters
                generatePermutation(str, start + 1, end);
                // Backtracking and swapping the characters again.
                str = swapString(str, start, i);
            }
        }
    }
}