cs: add “1768. Merge Strings Alternately”

Signed-off-by: Matej Focko <mfocko@redhat.com>

cs/merge-strings-alternately.cs

@@ -0,0 +1,20 @@
+public class Solution {
+    public string MergeAlternately(string word1, string word2) {
+        var sb = new StringBuilder();
+
+        int i, j;
+        for (i = 0, j = 0; i < word1.Length && j < word2.Length; ++i, ++j) {
+            sb.Append(word1[i]);
+            sb.Append(word2[j]);
+        }
+
+        if (i < word1.Length) {
+            sb.Append(word1.Substring(i));
+        }
+        if (j < word2.Length) {
+            sb.Append(word2.Substring(j));
+        }
+
+        return sb.ToString();
+    }
+}