Normal to recursive method due tomorrow please help!!

I need to make run() a recursive method, and i have no way of doing it. I tried many ways but cannot do it. Can someone please help me, it's due tomorrow and I've spent hours on it.

Program:
Write a program that prompts for a positive integer and displays all permutations of the odd digits of that length. Display these values in ascending order. For example, if the length is 3, your program should display:

111
113
115
117
119
131
133
135
. . .

^there shouldn't be an even digit in any number.

Code:

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  import java.util.Scanner;
  public class OddDigits
  {
      public static void main(String [] arg)
      {
        Scanner x = new Scanner(System.in);
     
        System.out.print("Enter a number: ");
        int a = x.nextInt();
        int b = 1;
       
        run(a, b);
      }
 
      public static boolean isOdd(int a)
      {
        int n =0;
        while (a > 1)
        {
            if(a%2 == 0)
              return false;
            a = a/10;
        }
         
        return true;
      }
     
      public static void run(int a, int b)
      {
        int upLim = (int)Math.pow(10,a);
        int lowLim = (int)Math.pow(10, a-1) -1;
       
        int start = (int)Math.pow(10, a-1);
       
        while(start < upLim)
        {
            if(isOdd(start) == true)
              System.out.println(start);
            start++;
        }
        //run(a, start++);
      }
 
  }

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