Recursion - digits in reverse order

Question

I need to implement a recursive method printDigits that takes an integer num as a parameter and prints its digits in reverse order, one digit per line.

This is what

Answer 1

public static int reversDigits(int num) {
    if(num < 1) {
        return 0;
    }

    int temp = num % 10;
    num = (num - temp)/10;
    System.out.println(temp);

    return reversDigits(num);
}

This will print the digits one at a time in reverse order. You don't need to do System.out in your main method.

Answer 2

I found I had to pick off the highest digit (on the left) and work towards the rightmost digit. I couldn't get a recursive one to work going from right to left.

public static int reverseItRecursive(int number)
{
    if (number == 0)
        return 0;

    int n = number;
    int pow = 1;

    while (n >= 10)
    {
        n = n / 10;
        pow = pow * 10;
    }
    return (n + reverseItRecursive(number - n*pow)*10);
}

Answer 3

This is very shortest/simplest way in two lines code:

    public static int reverseNumber(int n) 
    {
        System.out.println(n % 10);
        return (n/10 > 0) ? reverseNumber(n/10) : n;
    }

Answer 4

This method reverse the integer and returns the result without using any string functions, Math, or by just printing

public class ReverseNumber {

    public static void main (String[] args) {
        ReverseNumber rNumber = new ReverseNumber();
        System.out.println(rNumber.reverseRecursive(1234,0)); // pass zero to initialize the reverse number
    }
    public int reverseRecursive(int n, int reverse) {// n - the number to reverse
        // System.out.println(n);

        if (n != 0){
            reverse = reverse * 10;
            reverse = reverse + n %10;
            n = n/10;
        } else {
              return reverse;
        }
    return reverseRecursive(n,reverse);
}}

Answer 5

//Reverse a number using recursion by bibhu.rank
public class Rev_num {
    public static int revnum(int x){
        int temp1=x,temp2=1;
        if(x<10){
            return x;
        }
        while(temp1>=10){
            temp2*=10;
            temp1/=10;
        }
        if(((x%temp2) < (temp2/10))&& x%temp2!=0){
            int c=temp2;
            while(c> x%temp2){
                c/=10;
            }
            c=temp2/c;
            temp2=x%temp2;
            return((temp1)+(c*revnum(temp2)));
        }
        temp2=x%temp2;

        return (temp1+(10*revnum(temp2)));

    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("Enter a number");
        Scanner y=new Scanner(System.in);
        System.out.println(revnum(y.nextInt()));
        y.close();


    }

}

Answer 6

//Try out this, recursion with singe variable using Math class.
public static void main(String[] args) {
    // Let the number be 139
    int n=139;
    System.out.println("reverse is "+rev(n));
}
static int rev(int n){
    if (n==0)return 0;
    else {
        return n%10*(int) Math.pow(10,(double) (int)Math.log10(n))+rev(n/10);
    }    
}