class FileSystem {

private Trie root = new Trie();

public FileSystem() {

}

public List<String> ls(String path) {

return root.find(path.split("/"));

}

public void mkdir(String path) {

root.addDir(path.split("/"));

}

public void addContentToFile(String filePath, String content) {

root.addFile(filePath.split("/"), content);

}

public String readContentFromFile(String filePath) {

return root.retrieve(filePath.split("/"));

}

class Trie {

Map<String, Trie> children = new HashMap<>();

boolean isDirectory = true;

String name = "";

List<String> contents = new ArrayList<>();

private int startPos(String[] path) {

return (path.length > 0 && path[0].length() == 0) ? 1 : 0;

}

List<String> find(String[] path) {

return find(path, startPos(path));

}

List<String> find(String[] path, int i) {

if (path.length == i) {

if (isDirectory) {

List<String> list = new ArrayList<>(children.keySet());

Collections.sort(list);

return list;

} else {

List<String> list = new ArrayList<>();

list.add(name);

return list;

}

}

if (!children.containsKey(path[i])) {

return new ArrayList<>();

}

return children.get(path[i]).find(path, i+1);

}

void addDir(String[] path) {

addDir(path, startPos(path));

}

void addDir(String[] path, int i) {

name = path[i-1];

if (path.length == i) {

return;

}

children.computeIfAbsent(path[i], k -> new Trie()).addDir(path, i+1);

}

void addFile(String[] path, String content) {

addFile(path, startPos(path), content);

}

void addFile(String[] path, int i, String content) {

name = path[i-1];

if (path.length == i) {

isDirectory = false;

contents.add(content);

return;

}

children.computeIfAbsent(path[i], k -> new Trie()).addFile(path, i+1, content);

}

String retrieve(String[] path) {

return retrieve(path, startPos(path));

}

String retrieve(String[] path, int i) {

if (path.length == i) {

StringBuilder sb = new StringBuilder();

for (String c: contents) sb.append(c);

return sb.toString();

}

if (!children.containsKey(path[i])) {

return "";

}

return children.get(path[i]).retrieve(path, i+1);

}

}

}