Question

Here is some Java code to reverse a string recursively.

Could someone provide an explanation of how it works?

public static String reverse(String str) {
    if ((null == str) || (str.length() <= 1)) {
        return str;
    }
    return reverse(str.substring(1)) + str.charAt(0);
}

I’m not understanding how this can possibly work.

Answer 1

The function takes the first character of a String – str.charAt(0) – puts it at the end and then calls itself – reverse() – on the remainder – str.substring(1), adding these two things together to get its result – reverse(str.substring(1)) + str.charAt(0)

When the passed in String is one character or less and so there will be no remainder left – when str.length() <= 1) – it stops calling itself recursively and just returns the String passed in.

So it runs as follows:

reverse("Hello")
(reverse("ello")) + "H"
((reverse("llo")) + "e") + "H"
(((reverse("lo")) + "l") + "e") + "H"
((((reverse("o")) + "l") + "l") + "e") + "H"
(((("o") + "l") + "l") + "e") + "H"
"olleH"