Java find all permutations of string

In order to solve this problem, we need to understand the concept of backtracking.

According to the backtracking algorithm:

• Fix the character in the first position, and then replace the remaining characters with the first character. As in ABC, in the first iteration, three strings are formed by swapping A with A, B and C, respectively: ABC, BAC, and CBA.
• Repeat step 1 for the remaining characters, for example, fix the second character B, and so on.
• Now swap again to return to the previous position. For example, from ABC, we form ABC by fixing B again, then backtracking to the previous position and swapping B with C. Therefore, now we have ABC and ACB.
• Repeat these steps for BAC and CBA to obtain all permutations.

algorithm

1. Define a string.
2. Fix the roles and swap the remaining roles.
3. Call generatePermutation() for the remaining characters.
4. Backtrack and swap characters again.

solution

package Permutations;

public class PermuteString {
	// 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);
			}
		}
	}

	// 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);
	}
}