Substring with Concatenation of All Words

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) in s that is a concatenation of each word in words exactly 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 "barfoo" and "foobar" respectively.
The output order does not matter, returning [9,0] is fine too.

Example 2:

Input:
  s = "wordgoodgoodgoodbestword",
  words = ["word","good","best","word"]
Output: []

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

        List<Integer> r = new ArrayList<>();
        int sLen = s.length();
        int num = words.length;
        int wordLen = words[0].length();

        for (int i = 0; i < sLen - num * wordLen + 1; i++) {
            String sub = s.substring(i, i + num * wordLen);
            if (isConcat(sub, counts, wordLen)) {
                r.add(i);
            }
        }
        return r;
    }

    private boolean isConcat(String sub, Map<String, Integer> counts, int wordLen) {
        Map<String, Integer> seen = new HashMap<>();
        for (int i = 0; i < sub.length(); i += wordLen) {
            String sWord = sub.substring(i, i + wordLen);
            seen.put(sWord, seen.getOrDefault(sWord, 0) + 1);
        }
        return seen.equals(counts);
    }
}