Question

I wrote this code to find all possible permutations of some numbers. But i dosen’t want to use one digit twice:
123,132,213 are OK, but it produces numbers like 122, 121 etc.
What am i doing wrong?

import java.util.HashSet;

public class main {

public static void main(String[] args) {        

    HashSet<Integer> l = new HashSet<Integer>();        
    for(int i=0;i<=3;i++){
        l.add(i);
    }       
    perm(l,3,new StringBuffer());

}

 static void perm(HashSet<Integer> in, int depth,StringBuffer out){             
    if(depth==0){
        System.out.println(out);
        return;
    }       

    int len = in.size();
    HashSet<Integer> tmp = in;

    for(int i=0;i<len;i++){
        out.append(in.toArray()[i]);
        tmp.remove(i);

        perm(tmp,depth-1,out);

        out.deleteCharAt(out.length()-1);
        tmp.add(i);
    }
}
}

Answer 1

It looks like Autoboxing is getting you. When you call the remove with ‘i’, My guess is that ‘i’ has been boxed to a different object and is thus not found in your HashSet.