Reversing a string with recursion in Java

Question

Below is my Java code to reverse a string recursively:

public static String reverse(String str) {

    if ((null == str) || (str.length() <= 1)) {

        return str;

    }

    return reverse(str.substring(1)) + str.charAt(0);

}

Can anyone give me an explanation of how it works?

Answer 1

When you call the function, it takes the 1st character ‘ str.charAt(0)’ of the string and places it at the end. Then it calls the  itself - reverse() - on the remainder - str.substring(1). When you add it together, you get the result reverse(str.substring(1)) + str.charAt(0)

When the passed String has one character or less with no remainder left (str.length()  <= 1), then it stops calling itself recursively to return the passed string. 

See the below implementation to understand how it works:

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"

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