public class Solution
{
    public IList<int> FindSubstring(string s, string[] words)
    {
        var len = words[0].Length;
        var allLen = len * words.Length;
        var dict = new Dictionary<string, int>();
        foreach (var word in words)
        {
            dict.TryAdd(word, 0);
            dict[word]++;
        }
        var answer = new List<int>();
        var windowDict = new Dictionary<string, int>();
        for (var offset = 0; offset < len; offset++)
        {
            for (var i = offset; i <= s.Length-len; i += len)
            {
                var word = s[i..(i + len)];
                var prevInx = i - allLen;
                if (prevInx >= 0)
                {
                    var leftWord = s[prevInx..(prevInx + len)];
                    windowDict[leftWord]--;
                }
                windowDict.TryAdd(word, 0);
                windowDict[word]++;
                if (dict.All(kvp => windowDict.TryGetValue(kvp.Key, out var count) && count == kvp.Value))
                    answer.Add(prevInx + len);
            }
            windowDict.Clear();
        }
        return answer;
    }
}