30 Substring with Concatenation of All Words ***** Sliding Window

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> map = new HashMap<>();
        for (String word : words) {
            if (map.containsKey(word)) {
                map.put(word, map.get(word) + 1);
            }else{
                map.put(word,1);
            }
        }
        int n = words.length;
        int m = words[0].length();
        for (int i = 0; i <= s.length() - n * m; i++) {
            Map<String, Integer> copy = new HashMap<>(map);
            int j = i;
            int count = n;
            while (count > 0) {
                String temp = s.substring(j, j+m);
                if (!copy.containsKey(temp) || copy.get(temp) < 1) {
                    break;
                }
                copy.put(temp, copy.get(temp) - 1);
                count--;
                j = j + m;
            }
            if (count == 0) {
                ans.add(i);
            }
        }
        return ans;
    }
}