Design a data structure for a directory. The root directory is A, A has three subdirectories, B, C and D; B has three subdirectories, B1, B2 and B3; C has no subdirectories; D has two subdirectories, D1 and D2; B2 has two subdirectories, B21 and B22; D2 has one subdirectory, D21; D21has three subdirectories, D211,D212 and D213. Please draw the schematic diagram of the directory, and draw the schematic diagram of the storage structure designed for the directory. Please design an efficient algorithm to count the number of directories without subdirectories..