public List<Integer> findSubstring(String s, String[] words) {

final List<Integer> res = new ArrayList<>();

final Map<String, Integer> f = new HashMap<>();

for (String str : words) {

f.merge(str, 1, Integer::sum);

}

final int n = s.length();

final int m = words.length;

final int l = words[0].length();

for (int start = 0; start < l; start++) {

final Map<String, Integer> curr = new HashMap<>(f);

int matched = m;

int j = start;

for (int i = start; i <= n - l; i += l) {

if (curr.merge(s.substring(i, i + l), -1, Integer::sum) >= 0) {

matched--;

}

while (matched == 0) {

if (i - j + l == m * l) {

res.add(j);

}

if (curr.merge(s.substring(j, j + l), 1, Integer::sum) == 1) {

matched++;

}

j += l;

}

}

}

return res;

}