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class FileSystem {
    private static class Node {
        Map<String, Node> children;
        StringBuilder content;

        Node(Map<String, Node> _children) {
            this.children = _children;
        }

        Node(StringBuilder _content) {
            this.content = _content;
        }
    }

    private Node root;

    public FileSystem() {
        root = new Node(new TreeMap<>());
    }

    public List<String> ls(String path) {
        String[] intermediate = path.split("/");
        Node ptr = root;
        for (int i = 1; i < intermediate.length; i++) {
            ptr = ptr.children.get(intermediate[i]);
        }
        List<String> ans = new ArrayList<>();
        if (ptr.content != null) {
            ans.add(intermediate[intermediate.length - 1]);
        } else {
            for (String child : ptr.children.keySet()) {
                ans.add(child);
            }
        }
        return ans;
    }

    public void mkdir(String path) {
        Node ptr = root;
        String[] files = path.split("/");
        for (int i = 1; i < files.length; i++) {
            String file = files[i];
            ptr.children.putIfAbsent(file, new Node(new TreeMap<>()));
            ptr = ptr.children.get(file);
        }
    }

    public void addContentToFile(String filePath, String content) {
        Node ptr = root;
        String[] files = filePath.split("/");
        for (int i = 1; i < files.length - 1; i++) {
            ptr = ptr.children.get(files[i]);
        }
        if (ptr.children.containsKey(files[files.length - 1])) {
            ptr.children.get(files[files.length - 1]).content.append(content);
        } else {
            Node newFile = new Node(new StringBuilder());
            newFile.content.append(content);
            ptr.children.put(files[files.length - 1], newFile);
        }
    }

    public String readContentFromFile(String filePath) {
        Node ptr = root;
        String[] files = filePath.split("/");
        for (int i = 1; i < files.length - 1; i++) {
            ptr = ptr.children.get(files[i]);
        }
        return ptr.children.get(files[files.length - 1]).content.toString();
    }
}
/**
 * Your FileSystem object will be instantiated and called as such:
 * FileSystem obj = new FileSystem();
 * List<String> param_1 = obj.ls(path);
 * obj.mkdir(path);
 * obj.addContentToFile(filePath,content);
 * String param_4 = obj.readContentFromFile(filePath);
 */

每个file或者direcory都是一个node, linux中无论文件还是文件夹都认为是file, 那么这里无论file还是dir都抽象为一个node. node的content如果是null, 那么就是文件夹, 否则是文件.

添加文件, ls, mkdir, read就是沿着path走即可.