How to make permutation program using StringBuilder in java?

Question

I am willing to create a program that outputs all the possible permutations of a string. For example:

Input abc

Output

abc
acb
bac
bca
cab
cba

I am able to get this output when I use String in java, but I am curious do the same using StringBuilder in java.

I used the following code to do get this output.

public class Test {
    public static void permutations(String str, String ans) {
        if (str.length() == 0) {
            System.out.println(ans);
            return;
        }
        for (int i = 0; i < str.length(); i++) {
            String newStr = str;
            newStr = newStr.substring(0, i) + newStr.substring(i + 1);
            permutations(newStr, ans + str.charAt(i));
        }
    }

    public static void main(String[] args) {        
            permutations("abc", ""); // using String
    }
}

I tried to create a similar method that resembles in structure with my permutation method and takes StringBuilder as parameter and work for it, but I am unable to do so, it shows me StringIndexOutOfBound Exception etc.

I am sharing the method I built below.

    public static void permutationsStringBuilder(StringBuilder str, StringBuilder ans) {
        if (str.length() == 0) {
            System.out.println(ans);
            return;
        }
        StringBuilder newstr = new StringBuilder(str);
        for (int i = 0; i < str.length(); i++) {
            try {
                ans.append(str.charAt(i));
                newstr.deleteCharAt(i);
                permutationsStringBuilder(newstr, ans);
                ans.deleteCharAt(i);
                newstr.replace(i, i, ""+str.charAt(i));
            } catch (StringIndexOutOfBoundsException e) {
            }
        }
    }

Output this is returning is:

abc
ccb
bac
cca

Please help me with this. Assist me make it work using StringBuilder of Java.

score 1 · Accepted Answer · answered Jan 18 '23 at 14:41

After the recursive call, you need to reset each StringBuilder to its original state before the loop iteration.

One character was appended to ans, the last character should be removed.
The ith character of newstr was removed, so it should be added back at the correct position.

ans.append(str.charAt(i));
newstr.deleteCharAt(i);
permutationsStringBuilder(newstr, ans);
ans.deleteCharAt(ans.length() - 1);
newstr.insert(i, str.charAt(i));

How to make permutation program using StringBuilder in java?

1 Answers1