How to reverse a string using a recursive function?

Question

I've seen lots of posts about reversing a string with recursion, but I like to use my own style of coding to properly understand it. Anyway here is the code I have.

private static String reverse (String s){

    String rev = "";
    int index = s.length()-1;
    if(index >= 0){

        rev += s.charAt(index);
        index--;
        rev += reverse(rev);


    }
        return rev;

    }

Basically index just keeps going for every single character when index is -1 the loop stops. it reads the last possible character of the string but there is an error in this line here.

rev += rev(str)

Here is iterative method

String dog = "dog";
String rev = "";

int index = dog.length()-1;
while(index >= 0){
    rev+=dog.charAt(index);
    index--;

}

out.println(rev);

Answer 1

I believe when the method calls itself you reset the value of the index. Maybe a better approach would be to have an overloaded method where the original call passes in the string to process and then the remaining calls call a method where you pass in both the string you are building along with the remaining part of the that is being processed.

Answer 2

The recursive step doesn't contain the mutated string. You need to pass the substring of the original string. rev = reverse(s.substring(0,index));

Answer 3

(Posted answer on behalf of the OP) .

It's fixed now. Thank you so much, I was indexing too late and passing in negative numbers.

private static String reverse (String s) {

int index = s.length();
index--;
String rev = "";
if(index >= 0){
    rev += s.charAt(index);

    rev = rev + reverse(s.substring(0,index));
}

return rev;

}