String permutation

A permutation, also called an "arrangement number" or "order," is a rearrangement of the elements of an ordered list into a one-to-one correspondence with itself

Algorithm
To get all the permutations, we will first take out the first char from String and permute the remaining chars.

If String = “ABC”
First char = A and remaining chars permutations are BC and CB.

Now we can insert first char in the available positions in the permutations.
BC -> ABC, BAC, BCA
CB -> ACB, CAB, CBA

So we can write a recursive function call to return the permutations and then another function call to insert the first characters to get the complete list of permutations.

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import java.util.*; public class Permutations { public static Set<String> permutationFinder(String str) { Set<String> perm = new HashSet<String>(); //whitespaces if (str.length() == 0) { perm.add(""); return perm; } char initial = str.charAt(0); // first character String rem = str.substring(1); // Full string without first character Set<String> words = permutationFinder(rem); for (String strNew : words) { for (int i = 0;i<=strNew.length();i++){ perm.add(charInsert(strNew, initial, i)); } } return perm; } public static String charInsert(String str, char c, int j) { String begin = str.substring(0, j); String end = str.substring(j); return begin + c + end; } public static void main(String[] args) { String s = "1234"; System.out.println("\nPermutations for " + s + " are: \n" + permutationFinder(s)); } }