Removing an input from a recursive method

Question

Good morning! I received a problem statement to write a method that returns all possible combinations of a String input passed, e.g.

if ABC is passed then it returns [A, AB, BC, ABC, AC, B, C] if ABCD is passed then it returns [A, AB, BC, CD, ABC, AC, ACD, B, BCD, BD, ABD, AD, C, D, ABCD]

means AB and BA are always taken same, ABC, BAC and ACB are also same.

I ended up writing below code and it seems to working though (not sure).

public static Set<String> getAnyPermutations(String s,String strInput) {
    Set<String> resultSet = new HashSet<>();
    char[] inp = strInput.toCharArray();
    for(int i=0; i<inp.length; i++) {
        String temp =s+String.valueOf(inp[i]);
        resultSet.add(temp);
        if(i+1<=inp.length)
            resultSet.addAll(getAnyPermutations(temp, String.valueOf(Arrays.copyOfRange(inp, i+1, inp.length))));
    }
    return resultSet; 
}

My question is, I want to remove the first param(String s) from the method as using it for interal comutations only, or if that is not possible then making sure that user always pass a "" value or I can reset it to "" for the first(non-recursive) call of this method. I am going confused how to do that inside a recursive funtion. Also please add comment if you have doubt it can fail other than this situation.

Conditions, All has to be done inside this function only, no other method can be created.

Answer 1

All has to be done inside this function only, no other function can be created.

Then you can't do it. The function has no (reasonable)* way of knowing whether it called itself or was called by another function.

There are lots of solutions involving creating another function. One that might fit your requirements, depending on how they're actually expressed, would be to have the function define a lambda to do the work, and have the lambda call itself. E.g., getAnyPermutations wouldn't actually be recursive, it would contain a recursive function.

But that may be out of bounds depending on the exact meaning of the quote above, since the lambda is another function, just not one that can be accessed from the outside.

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* The unreasonable way is by examining a stack trace, which you can get from Thread.currentThread().getStackTrace.