diff --git a/30. Substring with Concatenation of All Words/Solution.cs b/30. Substring with Concatenation of All Words/Solution.cs
index 2ec9574..afabd4d 100644
--- a/30. Substring with Concatenation of All Words/Solution.cs	
+++ b/30. Substring with Concatenation of All Words/Solution.cs	
@@ -1,35 +1,34 @@
-public class Solution {
-    public IList<int> FindSubstring(string s, string[] words) {
+public class Solution
+{
+    public IList<int> FindSubstring(string s, string[] words)
+    {
         var len = words[0].Length;
-        var allLen = len*words.Length;
+        var allLen = len * words.Length;
         var dict = new Dictionary<string, int>();
-        foreach(var word in words)
+        foreach (var word in words)
         {
             dict.TryAdd(word, 0);
             dict[word]++;
         }
         var answer = new List<int>();
-        var dictClone = new Dictionary<string, int>(dict);
-        for(var i = 0; i < s.Length-allLen+1; i++)
+        var windowDict = new Dictionary<string, int>();
+        for (var offset = 0; offset < len; offset++)
         {
-            var valid = 0;
-            foreach (var kvp in dictClone)
-                dictClone[kvp.Key] = dict[kvp.Key];
-            for(var j=i; j < i+allLen; j+=len)
+            for (var i = offset; i <= s.Length-len; i += len)
             {
-                var word = s[j..(j+len)];
-                if(dictClone.TryGetValue(word, out var count) && count > 0)
+                var word = s[i..(i + len)];
+                var prevInx = i - allLen;
+                if (prevInx >= 0)
                 {
-                    dictClone[word]--;
-                    valid++;
-                }
-                else
-                {
-                    break;
+                    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);
             }
-            if(valid == words.Length)
-                answer.Add(i);
+            windowDict.Clear();
         }
         return answer;
     }