#588 Design In-Memory File System

1// On LeetCode, name this class FileSystem.2class FileSystemFlat {3    private final Set<String> dirs = new HashSet<>(Set.of("/"));      // every folder, as a full path4    private final Map<String, StringBuilder> files = new HashMap<>(); // file path → content5 6    public List<String> ls(String path) {7        if (files.containsKey(path)) return List.of(path.substring(path.lastIndexOf('/') + 1));8        String prefix = path.equals("/") ? "/" : path + "/";9        TreeSet<String> names = new TreeSet<>();             // sorted, no duplicates10        List<String> all = new ArrayList<>(dirs);11        all.addAll(files.keySet());12        for (String p : all) {                               // scans EVERY stored path: O(P · L)13            if (!p.startsWith(prefix) || p.length() == prefix.length()) continue;14            String rest = p.substring(prefix.length());15            int slash = rest.indexOf('/');16            names.add(slash < 0 ? rest : rest.substring(0, slash)); // the first name below path17        }18        return new ArrayList<>(names);19    }20 21    public void mkdir(String path) {22        for (int i = 1; i <= path.length(); i++) {           // "/a/b/c" adds /a, /a/b and /a/b/c23            if (i == path.length() || path.charAt(i) == '/') dirs.add(path.substring(0, i));24        }25    }26 27    public void addContentToFile(String filePath, String content) {28        files.computeIfAbsent(filePath, k -> new StringBuilder()).append(content);29    }30 31    public String readContentFromFile(String filePath) {32        return files.get(filePath).toString();33    }34}

1class FileSystem {2    private static class Node {3        final TreeMap<String, Node> children = new TreeMap<>(); // name → child, kept sorted for ls4        StringBuilder content;                                  // null for a folder, text for a file5    }6 7    private final Node root = new Node();8 9    /** Walks from the root, one name at a time, creating missing folders on the way. */10    private Node walk(String path) {11        Node node = root;12        for (String name : path.split("/")) { // "/a/b" → ["", "a", "b"]; "/" → []13            if (name.isEmpty()) continue;14            node = node.children.computeIfAbsent(name, k -> new Node());15        }16        return node;17    }18 19    public List<String> ls(String path) {20        Node node = walk(path);21        if (node.content != null) { // a file lists only its own name22            return List.of(path.substring(path.lastIndexOf('/') + 1));23        }24        return new ArrayList<>(node.children.keySet()); // TreeMap keys are already sorted25    }26 27    public void mkdir(String path) {28        walk(path); // creates every missing folder on the path29    }30 31    public void addContentToFile(String filePath, String content) {32        Node file = walk(filePath);33        if (file.content == null) file.content = new StringBuilder(); // a new file34        file.content.append(content); // append, never overwrite35    }36 37    public String readContentFromFile(String filePath) {38        return walk(filePath).content.toString();39    }40}