Add uppercase letters

Simple Fun 176 Reverse Letter/main.cpp

@@ -3,7 +3,7 @@
 
 std::string reverse_letter(const std::string &str)
 {
-    std::string alphabet ("abcdefghijklmnopqrstuvwxyz");
+    std::string alphabet ("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ");
     std::string result ("");
     for (int i = str.length() - 1; i >= 0; i--){
         if (alphabet.find(str[i]) != std::string::npos){