Dummy.java

import java.util.*;

public class Dummy {
    private static final Map<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();
    static {
        map.put(120, Arrays.asList(1));
        map.put(121, Arrays.asList(2, 3));
        map.put(122, Arrays.asList(4, 5, 6));
        // 0 - 0,0 | 1 - 1,0 | 2 - 1,1,
        // etc.
    }

    public static int getChildId(int index) {
        int sum = 0;
        List<Integer> keys = new ArrayList<Integer>(map.keySet());
        Collections.sort(keys);
        for (Integer key : keys) {
            int innerIndex = index - sum;
            List<Integer> innerList = map.get(key);
            int size = innerList.size();
            if (innerIndex < size) {
                return innerList.get(innerIndex);
            }
            sum += size;
        }
        return -1;// unknown or invalid index
    }

    // use this if you want to know how many items there are
    public static int getCount() {
        int sum = 0;
        for (Integer key : map.keySet()) {
            sum += map.get(key).size();
        }
        return sum;
    }
}