feat(solution): Added C# solutions for Two Sum, Contains Duplicate, and Group Anagrams

Author: maxchui

C#/1-Two_Sum.cs

@@ -0,0 +1,25 @@
+// URL: https://leetcode.com/problems/two-sum/
+// Time Complexity: O(n)
+// Space Complexity: O(n)
+
+public class Solution
+{
+    public int[] TwoSum(int[] nums, int target)
+    {
+        Dictionary<int, int> map = new Dictionary<int, int>();
+
+        for (int i = 0; i < nums.Length; i++)
+        {
+            int diff = target - nums[i];
+
+            if (map.ContainsKey(diff))
+            {
+                return new int[] { i, map[diff] };
+            }
+
+            map[nums[i]] = i;
+        }
+
+        return null;
+    }
+}

C#/217-Contains_Duplicate.cs

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+// URL: https://leetcode.com/problems/contains-duplicate/
+// Time Complexity: O(n)
+// Space Complexity: O(n)
+
+public class Solution
+{
+    public bool ContainsDuplicate(int[] nums)
+    {
+        return nums.Length != new HashSet<int>(nums).Count;
+    }
+}

C#/49-Group_Anagrams.cs

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+// URL: https://leetcode.com/problems/group-anagrams/
+// Time Complexity: O(n * m * log(m))
+// Space Complexity: O(n * m)
+
+public class Solution
+{
+    public IList<IList<string>> GroupAnagrams(string[] strs)
+    {
+        Dictionary<string, IList<string>> dict = new Dictionary<string, IList<string>>();
+
+        for (int i = 0; i < strs.Length; i++)
+        {
+            char[] arr = strs[i].ToCharArray();
+            Array.Sort(arr);
+            String orderedString = new String(arr);
+
+            if (!dict.ContainsKey(orderedString))
+            {
+                dict.Add(orderedString, new List<String>() { strs[i] });
+            }
+            else
+            {
+                dict[orderedString].Add(strs[i]);
+            }
+        }
+
+        return dict.Values.ToList();
+    }
+}