public static void main ( String [ ] args ) {
  Scanner scn = new Scanner ( System . in ) ;
  int n = scn . nextInt ( ) ;
  int [ ] [ ] adj = new int [ n ] [ n ] ;
  LinkedList < Edge > edges = new LinkedList < > ( ) ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    int v = scn . nextInt ( ) - 1 ;
    adj [ i ] [ v ] = 1 ;
    adj [ v ] [ i ] = 1 ;
    edges . add ( new Edge ( i , v ) ) ;
  }
  for ( Edge e : edges ) for ( int i = 0 ;
  i < n ;
  i ++ ) if ( adj [ i ] [ e . v ] == 1 && adj [ i ] [ e . u ] == 1 ) {
    System . out . println ( "YES" ) ;
    return ;
  }
  System . out . println ( "NO" ) ;
}

public static void main ( String [ ] args ) { Scanner scn = new Scanner ( System . in ) ; int n = scn . nextInt ( ) ; int [ ] [ ] adj = new int [ n ] [ n ] ; LinkedList < Edge > edges = new LinkedList < > ( ) ; for ( int i = 0 ; i < n ; i ++ ) { int v = scn . nextInt ( ) - 1 ; adj [ i ] [ v ] = 1 ; adj [ v ] [ i ] = 1 ; edges . add ( new Edge ( i , v ) ) ; } for ( Edge e : edges ) for ( int i = 0 ; i < n ; i ++ ) if ( adj [ i ] [ e . v ] == 1 && adj [ i ] [ e . u ] == 1 ) { System . out . println ( " YES " ) ; return ; } System . out . println ( " NO " ) ; }
(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: (array_type element: (integral_type) dimensions: (dimensions)) declarator: (variable_declarator name: (identifier) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (identifier)) dimensions: (dimensions_expr (identifier))))) (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)))) (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 (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) right: (decimal_integer_literal)))) (expression_statement (assignment_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: (identifier)) right: (decimal_integer_literal))) (expression_statement (assignment_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: (identifier)) right: (decimal_integer_literal))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (object_creation_expression type: (type_identifier) arguments: (argument_list (identifier) (identifier)))))))) (enhanced_for_statement type: (type_identifier) name: (identifier) value: (identifier) body: (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: (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: (field_access object: (identifier) field: (identifier))) right: (decimal_integer_literal)) right: (binary_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: (field_access object: (identifier) field: (identifier))) right: (decimal_integer_literal)))) consequence: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal)))) (return_statement))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (string_literal))))))