static int missingNumber ( int a [ ] , int n ) {
  Map < Integer , Integer > mp = new LinkedHashMap < > ( ) ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    if ( a [ i ] > 0 ) {
      mp . put ( a [ i ] , mp . get ( a [ i ] ) == null ? 1 : mp . get ( a [ i ] ) + 1 ) ;
    }
  }
  int index = 1 ;
  while ( true ) {
    if ( ! mp . containsKey ( index ) ) {
      return index ;
    }
    index ++ ;
  }
}

static int missingNumber ( int a [ ] , int n ) { Map < Integer , Integer > mp = new LinkedHashMap < > ( ) ; for ( int i = 0 ; i < n ; i ++ ) { if ( a [ i ] > 0 ) { mp . put ( a [ i ] , mp . get ( a [ i ] ) == null ? 1 : mp . get ( a [ i ] ) + 1 ) ; } } int index = 1 ; while ( true ) { if ( ! mp . containsKey ( index ) ) { return index ; } index ++ ; } }
(method_declaration (modifiers) type: (integral_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (integral_type) name: (identifier) dimensions: (dimensions)) (formal_parameter type: (integral_type) name: (identifier))) body: (block (local_variable_declaration type: (generic_type (type_identifier) (type_arguments (type_identifier) (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 (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (decimal_integer_literal))) consequence: (block (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (array_access array: (identifier) index: (identifier)) (ternary_expression condition: (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (array_access array: (identifier) index: (identifier)))) right: (null_literal)) consequence: (decimal_integer_literal) alternative: (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (array_access array: (identifier) index: (identifier)))) right: (decimal_integer_literal)))))))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) (while_statement condition: (parenthesized_expression (true)) body: (block (if_statement condition: (parenthesized_expression (unary_expression operand: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))))) consequence: (block (return_statement (identifier)))) (expression_statement (update_expression (identifier)))))))