Added a solution (C#) to 2215 find the difference of two arrays

csharp/0028-find-the-index-of-the-first-occurrence-in-a-string.cs

@@ -0,0 +1,33 @@
+public class Solution
+{
+    public int StrStr(string haystack, string needle)
+    {
+        if (needle.Length > haystack.Length) return -1;
+
+        int l = 0;
+        int r = needle.Length - 1;
+
+        while (r < haystack.Length)
+        {
+            bool areEqual = true;
+            for (int i = l, j = 0; i <= r; i++, j++)
+            {
+                if (haystack[i] != needle[j])
+                {
+                    areEqual = false;
+                    break;
+                }
+            }
+
+            if (areEqual)
+            {
+                return l;
+            }
+
+            l++;
+            r++;
+        }
+
+        return -1;
+    }
+}

csharp/2215-find-the-difference-of-two-arrays.cs

@@ -0,0 +1,26 @@
+public class Solution
+{
+    public IList<IList<int>> FindDifference(int[] nums1, int[] nums2)
+    {
+        HashSet<int> set1 = new HashSet<int>(nums1);
+        HashSet<int> set2 = new HashSet<int>(nums2);
+
+        IList<IList<int>> res = new List<IList<int>>() { new List<int>(), new List<int>() };
+
+        foreach (var el in set1)
+        {
+            if (set2.Contains(el)) continue;
+
+            res[0].Add(el);
+        }
+
+        foreach (var el in set2)
+        {
+            if (set1.Contains(el)) continue;
+
+            res[1].Add(el);
+        }
+
+        return res;
+    }
+}