Whats the best way to recursively reverse a string in Java?

I have been messing around with recursion today. Often a programming technique that is not used enough.

I set out to recursively reverse a string. Here's what I came up with:

//A method to reverse a string using recursion
    public String reverseString(String s){
        char c = s.charAt(s.length()-1);
        if(s.length() == 1) return Character.toString(c);   

        return c + reverseString(s.substring(0,s.length()-1));
    }

My question: is there a better way in Java?

Solution 1:

The best way is not to use recursion. These stuff are usually used to teach students the recursion concept, not actual best practices. So the way you're doing it is just fine. Just don't use recursion in Java for these kind of stuff in real world apps ;)

PS. Aside what I just said, I'd choose "" as the base case of my recursive function:

public String reverseString(String s){
    if (s.length() == 0) 
         return s;

    return reverseString(s.substring(1)) + s.charAt(0);
}

Solution 2:

If you're going to do this, you want to operate on a character array, because a String is immutable and you're going to be copying Strings all over the place if you do it that way.

This is untested and totally stream of consciousness. It probably has an OB1 somewhere. And very not-Java.

public String reverseString(String s)
  {
  char[] cstr = s.getChars();
  reverseCStr(cstr, 0, s.length - 1);

  return new String(cstr);
  }

/**
 * Reverse a character array in place.
 */
private void reverseCStr(char[] a, int s, int e)
  {
  // This is the middle of the array; we're done.
  if (e - s <= 0)
    return;

  char t = a[s];
  a[s] = a[e];
  a[e] = t;
  reverseCStr(a, s + 1, e - 1);
  }