Question

i have 2 sequences, for instance s=aaba and ss=aa, and i want all the way ss is in s.
In this example:
[0,1], [0,3] and [1,3]
My code is below. It works fine, except for very long s with multiple ss. In that case i’ve got

Exception in thread "main" java.lang.OutOfMemoryError: GC overhead limit exceeded
(I already use java with -Xmx at the maximum I can…)

public static ArrayList<ArrayList<Integer>> getListIndex(String[] s, String[] ss, int is, int iss) {
    ArrayList<ArrayList<Integer>> listOfListIndex = new ArrayList<ArrayList<Integer>>();
    ArrayList<ArrayList<Integer>> listRec = new ArrayList<ArrayList<Integer>>();
            ArrayList<Integer> listI = new ArrayList<Integer>();

    if (iss<0||is<iss){
        return listOfListIndex;
    }

    if (ss[iss].compareTo(s[is])==0){

        //ss[iss] matches, search ss[0..iss-1] in s[0..is-1]
        listRec = getListIndex(s,ss,is-1,iss-1);

        //empty lists (iss=0 for instance)
        if(listRec.size()==0){
            listI = new  ArrayList<Integer>();
            listI.add(is);
            listOfListIndex.add(listI);
        }
        else{
            //adding to what we have already found
            for (int i=0; i<listRec.size();i++){
                listI = listRec.get(i);
                    listI.add(is);
                    listOfListIndex.add(listI);
            }
        }
    }
    //In all cases
    //searching ss[0..iss] in s[0..is-1]
    listRec = getListIndex(s,ss,is-1,iss);
    for (int i=0; i<listRec.size();i++){
        listI = listRec.get(i);
            listOfListIndex.add(listI);
    }

    return listOfListIndex;
}   

Is there anyway to do this more efficiently ?

Answer 1

I doubt the recursion is the problem (think of what the maximum recursion depth is). The algorithm can be efficiently implemented by collecting the indecies of each character of s in ss in TreeSets and then simply taking the .tailSet when needing to “advance” in the string.

import java.util.*;


public class Test {

    public static Set<List<Integer>> solutions(List<TreeSet<Integer>> is, int n) {

        TreeSet<Integer> ts = is.get(0);
        Set<List<Integer>> sol = new HashSet<List<Integer>>();
        for (int i : ts.tailSet(n+1)) {
            if (is.size() == 1) {
                List<Integer> l = new ArrayList<Integer>();
                l.add(i);
                sol.add(l);
            } else 
                for (List<Integer> tail : solutions(is.subList(1, is.size()), i)) {
                    List<Integer> l = new ArrayList<Integer>();
                    l.add(i);
                    l.addAll(tail);
                    sol.add(l);
                }
        }
        return sol;
    }


    public static void main(String[] args) {
        String ss = "aaba";
        String s = "aa";

        List<TreeSet<Integer>> is = new ArrayList<TreeSet<Integer>>();

        // Compute all indecies of each character.
        for (int i = 0; i < s.length(); i++) {
            TreeSet<Integer> indecies = new TreeSet<Integer>();
            char c = s.charAt(i);
            for (int j = 0; j < ss.length(); j++) {
                if (ss.charAt(j) == c)
                    indecies.add(j);
            }
            is.add(indecies);
        }

        System.out.println(solutions(is, -1));
    }
}

Output:

[[0, 1], [1, 3], [0, 3]]