reverse string using recursion

Question

Could anyone please explain why "return str" line never executes?

public static String reverseString(String str){
    String reverse="";
    if(str.length() == 1){
        return str; //at one point this condition will be true, but value never returns
    } else {
        reverse += str.charAt(str.length()-1) + reverseString(str.substring(0,str.length()-1));
        return reverse;
    }
}

public static void main(String a[]) {
    System.out.println(reverseString("Test"));
}

Answer 1

The line does executes, how can you say it does not executes. I have added the syso statement, it does print, actually you are calling substring in a recursion, once the length becomes 1, it will execute.

public static String reverseString(String str) {
        String reverse = "";
        if (str.length() == 1) {
            System.out.println("hi");
            return str; // at one point this condition will be true, but value never returns
        } else {
            reverse += str.charAt(str.length() - 1) + reverseString(str.substring(0, str.length() - 1));
            return reverse;
        }
    }

    public static void main(String a[]) {
        System.out.println(reverseString("Test"));
    }

output

hi
tseT

Answer 2

You can easily use StringBuilder#reverse method

public String reverser(String str){
   return new StringBuilder(str).reverse().toString();
}

Answer 3

public static String reverseString(String str) {
    String reverse = "";
    if (str.length() == 1) {
        return str; // at one point this condition will be true, but value
                    // never returns
    } else {
        String part = reverseString(str.substring(0, str.length() - 1));
        System.out.println("Current: " + part); // Print out
        reverse = str.charAt(str.length() - 1)
                + part;
        return reverse;
    }
}

public static void main(String a[]) {
    System.out.println(reverseString("Test"));
}

Just add a print within your recursive function, to trace what is happening.

Output:

Current: T      // return str
Current: eT     // return reverse
Current: seT    // return reverse
tseT            // final return reverse

From the output, you can convince yourself whether str is being returned.

Answer 4

My Implemention:

    public static String reverse(String str) {
        if(str.length() > 1) {
            return str.substring(str.length()-1)+reverse(str.substring(0, str.length()-1));
        } else {
            return str;
        }
    }

    public static void main(String[] args) {
        System.out.println(reverse("Test"));
    }

output:

tseT

Answer 5

Actually it hits and executes, check this out;

public static String reverseString(String str){
    String reverse="";
    if(str.length() == 1 ){
        System.out.println("HIT: " + str);  // CHECKING HIT
        return str; //at one point this condition will be true, but value never returns
    } else {
        reverse += str.charAt(str.length()-1) + reverseString(str.substring(0,str.length()-1));
        return reverse;
    }
}

public static void main(String a[]) {
    System.out.println(reverseString("Abcd"));
}

If you run this code, you will see the output as below;

HIT: A
dcbA

To Understand how this code is working, you have to understand how reverse method call itself and completes its process;

Check the image down below;

As you can see, on the 3rd step, because that the recursive function's input string length equals to 1, that part of the code is executed.

Hope that all these helps.