new file:   combinations/combinations.py
        new file:   jump_game/solution.py
        new file:   minimum_depth_of_binary_tree/solution.py
        new file:   subsets/solution.py
        new file:   subsets_ii/solution.py
        new file:   sum_root_to_leaf_numbers/solution.py
        new file:   unique_path_ii/solution.py