You are given a string,s, and a list of words,words, that are all of the same length. Find all starting indices of substring(s) insthat is a concatenation of each word inwordsexactly once and without any intervening characters.

Example 1:

Input:
  s =
 "barfoothefoobarman",

  words = 
["foo","bar"]

Output:
[0,9]
Explanation:
 Substrings starting at index 0 and 9 are "barfoor" and "foobar" respectively.
The output order does not matter, returning [9,0] is fine too.

Example 2:

Input:
  s =
 "wordgoodstudentgoodword",

  words = 
["word","student"]

Output:
[]

class Solution {
    public List<Integer> findSubstring(String s, String[] words) {
        List<Integer> ans = new ArrayList<>();
        if (s == null || words == null || words.length == 0) return ans;
        Map<String, Integer> hash = new HashMap<>();
        for (String word : words){
            hash.put(word, hash.getOrDefault(word, 0) + 1);
        }


        int len = words[0].length();
        for (int i = 0; i <= s.length() - words.length * len; i++){
            Map<String, Integer> copy = new HashMap<>(hash);
            for (int j = 0; j < words.length; j++){
                String temp = s.substring(i + j * len, i + j * len + len);
                if (copy.containsKey(temp)){
                    int count = copy.get(temp);
                    if (count == 1){
                        copy.remove(temp);
                    }
                    else{
                        copy.put(temp, copy.get(temp) - 1);    
                    }
                }
                else break;
            }

            if (copy.size() == 0) ans.add(i);
        }

        return ans;
    }
}