new file:   best_time_to_buy_and_sell_stock/solution.py
        new file:   "binary_tree_preorder/\\"
        new file:   binary_tree_preorder/solution.py
        new file:   binary_tree_preorder/solution.pyc
        new file:   binary_tree_preorder/test.py
        new file:   binary_tree_preorder/test.pyc
        new file:   solution.py