Substring with Concatenation of All Words

import java.util.Hashtable;

public class Solution {
    public List<Integer> findSubstring(String s, String[] words) {
        List<Integer> rst = new ArrayList<Integer>();
        if(s==null || s.length()==0 || words==null || words.length==0)  return rst;
        int wordLen = words[0].length();
        int count = words.length;
        Hashtable<String,Integer> ht = new Hashtable<String,Integer>();
        for(String word: words)
        {
            if(ht.containsKey(word)) ht.put(word, ht.get(word)+1);
            else ht.put(word,1);    
        }
        for(int i=0; i<=s.length()-count*wordLen; i++)
        {
            String sub = s.substring(i,i+count*wordLen);
            if(isValid(sub, ht, i, wordLen)) rst.add(i);
        }
        return rst;
    }
    public boolean isValid(String sub, Hashtable<String,Integer> ht, int start, int wordLen)
    {
        Hashtable<String,Integer> found = new Hashtable<String,Integer>();
        for(int i=0; i<=sub.length()-wordLen; i+=wordLen)
        {
            String key = sub.substring(i,i+wordLen);
            if(ht.containsKey(key))
            {
                if(found.containsKey(key))
                {
                    found.put(key,found.get(key)+1);
                    if(found.get(key) > ht.get(key))    return false;
                }
                else found.put(key,1);
            }
            else return false;
        }
        return true;
    }
}