How can I implement that in my backtracker (java)?

Question

I have made a small java program. An integer will be an input and then random numbers will be printed to the console till we find that integer input. The code will cheat a bit since it will know how many digits the input number has.

Random numbers will be used and what I want to do is I want to exclude every random number that has been used. But I have no idea how it could be done. Well I have one idea but it would be a cheap bypass which I don't like. It would be by start at 0 and always increase it by 1 value till we find the input.

Could this (exclude used random number) be done in java?

I imagine we could make a list so every used number will be in that list. In every loop access the list to check if number is unused. But this is just imagination, would it really be possible in java?

import java.util.Scanner;
import java.util.Random;

public class Backtracker{
    public static void main(String[]args){
        int counter = 0;
        Scanner input = new Scanner(System.in);
        System.out.println("Enter integer here: ");
        int x = input.nextInt();
        Random rand = new Random();
        int a = rand.nextInt(100)+0;
        int b = rand.nextInt(1000)+100;
        int c = rand.nextInt(10000)+1000;
        int d = rand.nextInt(100000)+10000;
        int e = rand.nextInt(1000000)+10000;
        int f = rand.nextInt(10000000)+1000000;
        int g = rand.nextInt(100000000)+10000000;

        if(x<=100){
            while(x != a){
                a = rand.nextInt(100)+0;
                System.out.println(a);
                counter++;
            }
            System.out.println("Tries: "+counter);
        }

        else if(x<=1000){
            while(x != b){
                b = rand.nextInt(1000)+100;
                System.out.println(b);
                counter++;
            }
            System.out.println("Tries: "+counter);
        }

        else if(x<=10000){
            while(x != c){
                c = rand.nextInt(10000)+1000;
                System.out.println(c);
                counter++;
            }
            System.out.println("Tries: "+counter);
        }

        else if(x<=100000){
            while(x != d){
                d = rand.nextInt(100000)+10000;
                System.out.println(d);
                counter++;
            }
            System.out.println("Tries: "+counter);
        }

        else if(x<=1000000){
            while(x != e){
                e = rand.nextInt(1000000)+100000;
                System.out.println(e);
                counter++;
            }
            System.out.println("Tries: "+counter);
        }

        else if(x<=10000000){
            while(x != f){
                f = rand.nextInt(10000000)+1000000;
                System.out.println(f);
                counter++;
            }
            System.out.println("Tries: "+counter);
        }

        else if(x<=100000000){
            while(x != g){
                g = rand.nextInt(100000000)+10000000;
                System.out.println(g);
                counter++;
            }
            System.out.println("Tries: "+counter);
        }
    }
}

Answer 1

Using Random numbers doesn't sound like a good solution nor counting from 1. A good and simple enough solution could be:

a) Given a constraint that the input n must be 0

b) use basic binary search .

Since this sounds like an assignment so I won't write the code for this solution.