Permutation algorithm for array of integers in Java

Question

I have a working example to generate all char permutations in a String as below:

static ArrayList permutations(String s) {
        if (s == nul         


        

Answer 1

Here you go, the below sample code uses the recursive method to get the permutation. It is generic and you can specify the output location as you like. One bonus is you can specify delimiter as well.

import java.io.FileNotFoundException;
import java.io.OutputStream;
import java.io.PrintStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Permutation {
    //The entry of the permutation method
    public static  List permute(T[] arr){
        List result = new ArrayList();
        permute(new ArrayList(), Arrays.asList(arr), result);
        return result;
    }

    //This is the actual method doing the permutation
    private static  void permute(List pre, List cur, List out){
        int size = cur.size();

        if(size == 0){
            out.add((T[])pre.toArray());
        } else {
            for(int i=0; i tmpPre = new ArrayList(pre);
                List tmpCur = new ArrayList(cur);
                tmpPre.add(cur.get(i));
                tmpCur.remove((T)cur.get(i));

                permute(tmpPre, tmpCur, out);       
            }
        }
    }

    //Print each row of the permutated values
    private static  void print(List list, OutputStream out, char delim){
        try{
            for(T[] i : list){
                int count = 0;
                for(T t : i){
                    if(++count == i.length){
                        out.write((t.toString()).getBytes());
                    } else{
                        out.write((t.toString()+delim).getBytes());
                    }
                }
                out.write("\n".getBytes());
            }
        } catch (Exception ex){
            ex.printStackTrace();
        }
    }

    public static void main(String[] args) throws FileNotFoundException {
        Integer[] ints = new Integer[] {1, 2, 3, 4};
        Permutation.print(Permutation.permute(ints), System.out, ',');

        Character[] chars = {'a', 'b', 'c', 'd', 'e'};
        Permutation.print(Permutation.permute(chars), new PrintStream("permute.txt"), ' ');

        String[] strs = {"abc", "123"};
        Permutation.print(Permutation.permute(strs), System.err, ' ');
    }
}