Reverse String in Java without using any Temporary String,Char or String Builder

Question

Is it possible to reverse String in Java without using any of the temporary variables like String, Char[] or StringBuilder?

Only can use int, or int[].

Answer 1

String reverseMe = "reverse me!";
for (int i = 0; i < reverseMe.length(); i++) {
    reverseMe = reverseMe.substring(1, reverseMe.length() - i)
        + reverseMe.substring(0, 1)
        + reverseMe.substring(reverseMe.length() - i, reverseMe.length());
 }
 System.out.println(reverseMe);

Output:

!em esrever

Just for the fun of it, of course using StringBuffer would be better, here I'm creating new Strings for each Iteration, the only difference is that I'm not introducing a new reference, and I've only an int counter.

Answer 2

The objects of the Java String class are immutable - their contents cannot be altered after being created.

You will need at least two temporary objects - one for the final result and one for the intermediate values - even if you do find a way to avoid using a local variable.

EDIT:

That said, since you can use int[] you may be able to cheat.

Since char can be assigned to int, you can use String.charAt() to create an int array with the character values in reverse order. Or you may be allowed to use String.toCharArray() to get a char array that will be copied over to your int[] temporary.

Then you use the variable that holds the reference to your original string (or the result variable, if you are allowed one) to start from an empty string (easily obtainable with a direct assignment or String.substring()) and use String.concat() to create the final result.

In no case, however, will you be able to swap the characters in-place as you would do in C/C++.

EDIT 2:

Here's my version which does not use StringBuffer/Builders internally:

int r[] = new int[s.length()];

int idx = r.length - 1;

for (int i : s.toCharArray()) {
    r[idx--] = i;
}

s = s.substring(0, 0);

for (int i : r) {
    s = s.concat(String.valueOf((char)i));
}