function go() {
    var path = "//main//parent//child//grandchild”;
    var folder = getDriveFolder(path);
    Logger.log(folder.getUrl());
}

function getDriveFolder(path) {

  var name, folder, search, fullpath;

  // Remove extra slashes and trim the path
  fullpath = path.replace(/^\/*|\/*$/g, '').replace(/^\s*|\s*$/g, '').split("/");

  // Always start with the main Drive folder
  folder = DriveApp.getRootFolder();

  for (var subfolder in fullpath) {

    name = fullpath[subfolder];
    search = folder.getFoldersByName(name);

    // If folder does not exit, create it in the current level
    folder = search.hasNext() ? search.next() : folder.createFolder(name);

  }

  return folder;

}