Snippet to redirect single result search in WP.

redirect-single-result-search.php

 /**
  * Redirect single result search in WP.
  * @author  Everaldo Matias <matiaseveraldo@gmail.com>
  */

add_action( 'template_redirect', 'redirect_single_result_search' );
function redirect_single_result_search() {
    if ( is_search() ) {
        global $wp_query;
        if ( $wp_query->post_count == 1 ) {
          wp_redirect( get_permalink( $wp_query->posts['0']->ID ) );
          exit;
        }
    }
}