CODEWITHSUNDEEP
javapermutation
gethan
gethan

Reputation: 103

Find all permutations code Problem (Java)

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);
    }
}
}

Upvotes: 0

Views: 1364

Answers (3)

Matt
Matt

Reputation: 83

Here's a much simpler implementation using a HashSet. I'm using strings but the concept remains the same.

public void printPermutations(String str){
    HashSet<String> hs = new HashSet<>();
    printPermutations("", str, hs);
}

public void printPermutations(String prefix, String end, HashSet<String> hs){
    if(end.equals(""))
        System.out.println(prefix);
    for(int i = 0; i < end.length(); ++i)
        if(hs.add(prefix + end.charAt(i))){
            printPermutations(prefix + end.charAt(i), 
                    end.substring(0,i) + end.substring(i + 1, end.length()), hs);
        }

}

Upvotes: 0

Wer2
Wer2

Reputation: 86

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.

Upvotes: 1

Patrick87
Patrick87

Reputation: 28292

tmp.remove(i) is wrong. You need to remove the ith element from tmp... you are removing the element "i". So, do tmp.remove(in.toArray()[i]). I think that will fix this up. For instance, if the zeroth element is 17, doing tmp.remove(i) will remove all zeroes from the HashSet, not "17".

Upvotes: 2

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