Add Duplicate Character Counter in Dart (#4980)

Author: gangaasoonu

archive/d/dart/duplicate-character-counter.dart

@@ -0,0 +1,30 @@
+//Issue 4974
+void main(List<String> args){
+  const String error_message = "Usage: please provide a string";
+  if (args.isEmpty || args[0].isEmpty){
+    print(error_message);
+    return;
+  } // end of empty check
+  
+  Map<String, int> frequency = {};
+  bool duplicate_not_found = true;
+
+  for (int i = 0; i < args[0].length; i ++){
+    String char = args[0][i];
+    if (RegExp(r'[a-zA-Z0-9]').hasMatch(char)) {
+      frequency[char] = (frequency[char] ?? 0) + 1;
+    }
+  } // end of checking duplicate characters
+  
+  //either print all duplicate letters or set flag duplicate letters not found
+  frequency.forEach((char, count){
+    if (count > 1) {
+      duplicate_not_found = false; // at least 1 duplicate letter is found
+      print('$char: $count');
+    }
+  });
+  // print if no duplicate characters
+  if(duplicate_not_found == true){
+      print("No duplicate characters");
+  }
+}