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

Question

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 cam

Answer 1

In Java, since the String is immutable, the String concatenation would be more complex than it looks like.

For every concatenation, it creates a new string copying the contents of original String resulting in a linear complexity O(n) where n is the length of the string, so for m such operations it is O(m*n), we can say it is of quadratic complexity O(n^2).

We can use a StringBuilder which has O(1) complexity for each append. Below is the recursive program using StringBuilder. This uses only n/2 stack frames, so it has less space complexity than the normal recursive call which would be like s.charAt(s.length-1) + reverse(s.subString(0, s.length-2);

public class StringReverseRecursive {
    public static void main(String[] args) {
        String s = "lasrever gnirts fo noitatnemelpmi evisrucer a si sihT";
        StringBuilder sb = new StringBuilder(s);
        reverse(s, sb, 0, sb.length() - 1);
        System.out.println(sb.toString());
    }

    public static void reverse(String s, StringBuilder sb, int low, int high) {
        if (low > high)
            return;
        sb.setCharAt(low, s.charAt(high));
        sb.setCharAt(high, s.charAt(low));
        reverse(s, sb, ++low, --high);
    }
}