update largest_rectangle_in_histagram more simple
update binary_tree_preorder_traversal another solution
update binary_tree_zigzag_level_order_traversal another solution
update balanced_binary_tree more simple
update flatten_binary_tree_to_linked_list another solution
update populating_next_right_pointers_in_each_node_ii more simple
update unique_binary_search_trees another solution
update unique_binary_search_trees_ii
update construct_binary_tree a little fix
update validate_binary_search_tree more simple
update convert_sorted_array_to_binary_search_tree a little fix