public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int n = sc . nextInt ( ) ;
  int l = sc . nextInt ( ) ;
  ArrayList < String > list = new ArrayList < > ( ) ;
  sc . nextLine ( ) ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    list . add ( sc . nextLine ( ) ) ;
  }
  Collections . sort ( list , Comparator . naturalOrder ( ) ) ;
  StringBuilder sb = new StringBuilder ( ) ;
  for ( String str : list ) {
    sb . append ( str ) ;
  }
  System . out . println ( sb . toString ( ) ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int n = sc . nextInt ( ) ; int l = sc . nextInt ( ) ; ArrayList < String > list = new ArrayList < > ( ) ; sc . nextLine ( ) ; for ( int i = 0 ; i < n ; i ++ ) { list . add ( sc . nextLine ( ) ) ; } Collections . sort ( list , Comparator . naturalOrder ( ) ) ; StringBuilder sb = new StringBuilder ( ) ; for ( String str : list ) { sb . append ( str ) ; } System . out . println ( sb . toString ( ) ) ; }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (array_type element: (type_identifier) dimensions: (dimensions)) name: (identifier))) body: (block (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (field_access object: (identifier) field: (identifier)))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (generic_type (type_identifier) (type_arguments (type_identifier))) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (generic_type (type_identifier) (type_arguments)) arguments: (argument_list)))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (block (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))))))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier) (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list)))) (enhanced_for_statement type: (type_identifier) name: (identifier) value: (identifier) body: (block (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier)))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))))