inspired by https://github.com/pjones/ido-select-window

helm-select-window.el

(defun helm-window-init ()
  (let* ((wins (cdr (window-list)))
         (mapping (mapcar
                   (lambda (w) (cons (buffer-name (window-buffer w)) w))
                   wins)))
    (if (> (length wins) 1)
        ;; There are more than 2 windows in this frame.
        (setq helm-window-list-cache mapping)
      ;; There are 2 or fewer windows in this frame.
      (call-interactively 'other-window)
      (helm-exit-minibuffer))))

(defvar helm-source-window
  '((name . "Window")
    (init . helm-window-init)
    (candidates . helm-window-list-cache)
    (action . (("" . select-window)))))

;;;###autoload
(defun helm-select-window ()
  (interactive)
  (helm :sources '(helm-source-window)
        :buffer "*helm select window*"))

(provide 'helm-select-window)