Add a left bar button item to UINavigationController when no back button is present

- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated {
    if(navigationController.viewControllers.count != 1) { // not the root controller - show back button instead
        return;
    }
    UIBarButtonItem *menuItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemOrganize
                                                                              target:self
                                                                              action:@selector(menuItemSelected:)];   
    [viewController.navigationItem setLeftBarButtonItem:menuItem]; 
}

Swift code to add leftBarButtonItem…

    let leftMenuItem = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.Done, target: self, action: "leftMenuItemSelected:");
    self.navigationItem.setLeftBarButtonItem(leftMenuItem, animated: false);

Update

For Swift 4 & above

    let leftMenuItem = UIBarButtonItem(barButtonSystemItem: .done, target: self, action: #selector(leftMenuItemSelected(_:)))
    self.navigationItem.setLeftBarButton(leftMenuItem, animated: false);

Swift 3 version:

let done = UIBarButtonItem(barButtonSystemItem: .done, target: self, action: #selector(YourController.done))
navigationItem.setLeftBarButtonItem(done, animated: false)