Print all possible combinations of r elements using string

Question

I was trying to solve this question using this simple recursive and backtracking approach.My idea was this:

code

import java.util.*;

public class Main
{

    static void generate(String s, String ans , int r)
    {
        if(ans.length()-1==r)
        {
            System.out.print(ans);

            return;
        }

        char ch=s.charAt(0);

        for(int i=1;i<=s.length()-1;i++)
        {
            String ros=s.substring(i);

            generate(ros,ans+ch,r);
        }
    }

    public static void main(String[] args) {
        String a="123";
        Scanner sc=new Scanner(System.in);
        int r=sc.nextInt();
        String ans=" ";
        generate(a,ans,r);
    }
}

my output with string 123 : 12

Required output : 12 13 23

I hope my recursive tree is right but I am not able to code it. My code seems to have some major issues

By 'combinations', do you specifically mean 'of length 2'? 'Cause there's '', '1', '2', '3', '123' which are combinations of those not included in your intended output. — BeUndead, Mar 05 '21 at 13:10
And to check, does input with repeated characters count as different combinations. For example, for '111' as input, would you want '11', '11', '11' as output, or simply '11'? — BeUndead, Mar 05 '21 at 14:13

score 2 · Answer 1 · answered Mar 05 '21 at 13:58

if my understanding is right, input "12345" and 3, output should be

[1, 2, 3]
[1, 2, 4]
[1, 2, 5]
[1, 3, 4]
[1, 3, 5]
[1, 4, 5]
[2, 3, 4]
[2, 3, 5]
[2, 4, 5]
[3, 4, 5]

then this is my program

import java.util.*;


public class Main{
    static void generate(String s, int r){
        recursiveGenerate(s, new LinkedList<>(), r, 0);
    }

    static void recursiveGenerate(String s, Deque<Character> cache, int len, int start){
        if(cache.size() == len){
            System.out.println(Arrays.toString(cache.toArray()));
        }
        else{
            for(int i = start; i < s.length(); i++){
                cache.addLast(s.charAt(i));
                recursiveGenerate(s, cache, len, i + 1);
                cache.removeLast();
            }
        }
    }

    public static void main(String[] args){
        String a = "12345";
        int r = 3;
        generate(a, r);
    }
}

score 0 · Accepted Answer · answered Mar 07 '21 at 15:09

Here is different code using ArrayList but using same logic as **Nathan Drake** :

import java.util.*;

public class Main

{

    public static void PrintAllCombinations(int[] arr, int r) {
List<Integer> list = new ArrayList<Integer>();
PrintAllCombinationsUtils(arr, r, list, 0);
}

private static void PrintAllCombinationsUtils(int[] arr, int r, List<Integer> list, int start) {

if (list.size() >= r) {

System.out.println(list);

return;

}

for (int i = start; i < arr.length; i++) {

list.add(arr[i]);

PrintAllCombinationsUtils(arr, r, list, i + 1);

list.remove(list.size() - 1);

}

}

    public static void main(String[] args){

        Scanner sc=new Scanner(System.in);

        int r=sc.nextInt();

        int arr[]=new int[5];

        for(int i=0;i<arr.length;i++)

        {
            arr[i]=sc.nextInt();

        }

        PrintAllCombinations( arr, r);

    }
}

Print all possible combinations of r elements using string

2 Answers2