使用递归JAVA生成字符串中所有字符子集

Question

Forgive me if I mess this up, this is my first question. I've been working on this problem for hours. It is supposed to generate all subsets of characters ( not necessarily substrings ) in a string, using recursion. I commented it a lot so you could see my thinking and hopefully tell me where I'm going wrong. I'm using Eclipse as an IDE if that makes any difference.

 import java.util.ArrayList;

//Generates subsets of a string
public class SubsetGenerator
{
    private String original;
    private String remaining;
    private ArrayList<String> subsets;
    //Constructs a subset generator
    //@param input string to have subsets generated
    public SubsetGenerator(String input)
    {
        original = input;
        remaining = original;
        subsets = new ArrayList<String>();
    }

    public void printSubsets()
    {
        System.out.print(subsets);
    }
    //gets subsets
    public void generateSubsets()
    {       
        //if the string is empty, it has no subsets
        if(remaining.length() == 1)
        {
            subsets.add(remaining);
            return;
        }
        else
        {
            //remove the first character and hold onto it
            String removed = remaining.substring(0,1);
            remaining = remaining.substring(1);
            //recursion. Eventually it should add the last character in the string to the ArrayList and return
            generateSubsets();
            //Take each element that is in the ArrayList, add the removed character to it, add this back to the list
            for (int i = 0; i < subsets.size(); i++)
            {
                String temp = removed + subsets.get(i);
                subsets.add(temp);
            }
            subsets.add(removed);//add the removed character by itself
            return;
        }
    }

}

These are the errors I'm getting:

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at java.util.Arrays.copyOfRange(Arrays.java:3221)
    at java.lang.String.<init>(String.java:233)
    at java.lang.StringBuilder.toString(StringBuilder.java:447)
    at SubsetGenerator.generateSubsets(SubsetGenerator.java:41)
    at SubsetGenerator.generateSubsets(SubsetGenerator.java:37)
    at SubsetGenerator.generateSubsets(SubsetGenerator.java:37)
    at SubsetGenerator.generateSubsets(SubsetGenerator.java:37)
    at SubsetGeneratorTester.main(SubsetGeneratorTester.java:7)

I've tested it with this code:

public class SubsetGeneratorTester 
{
    public static void main(String[] args) 
    {
        SubsetGenerator s = new SubsetGenerator("world");
        s.generateSubsets();
        s.printSubsets();
    }

}

Answer 1

Here ya go:

public class SubsetGenerator {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ArrayList<String> list = new ArrayList<String>();
        StringBuilder myBuilder = new StringBuilder();
        String original = "World";


        for(int i=0; i<original.length(); ++i)
            genSubs(original, myBuilder, list, i);

        System.out.println(list.toString());


    }

static void genSubs(String original, StringBuilder current, ArrayList<String> myList, int index){

    current.append(original.charAt(index));

    System.out.println(current.toString() + index);

    myList.add(current.toString());


    for(int i=index+1; i<original.length(); ++i)
        genSubs(original, current, myList, i);

    current.deleteCharAt(current.toString().length()-1);


    return;
}

}

Output:

[W, Wo, Wor, Worl, World, Word, Wol, Wold, Wod, Wr, Wrl, Wrld, Wrd, Wl, Wld, Wd, o, or, orl, orld, ord, ol, old, od, r, rl, rld, rd, l, ld, d]

Answer 2

You should pass the remaining string as argument to generateSubsets(remaining) method. You are accessing the same copy of remaining on every recursion call and it always equal to original and it never hit the condition where remaining.length() == 1. You should pass the modified remaining string to generateSubsets().

Edit yes you are right. I found the problem. It's in your for loop. Every time you are adding an item to your subset, it size keep increasing. So for loop becomes infinite loop. Make this change

int size = subset.size()

for(int i =0; i< size; i++)
{..}

Answer 3

I would rather recommend to create characters array from the initial String and use the array. Creating plenty of substrings is no the best choice here.

Also you can increase available memory passing jvm arguments eg -Xmx1g -XX:MaxPermSize=512m -XX:MaxHeapSize=256m

Answer 4

I'm not quite sure what result you are looking for .... but if this is not it I am sure I can get it the way you want. Please give us a example data and result.

public void GetSubsets(String str, ArrayList list)
{
        if(str.length() > 0 && list != null)
        {
            list.add(str);
            GetSubsets(str.substring(1), list);
        }
}

the output for "World" is:

ArrayList list = new ArrayList();
GetSubsets("World", list);
System.out.println(list.toString());

"[World, orld, rld, ld, d]"

This code also does nothing when there is a empty string sent, or null list. edit out the list check if you please.