design-in-memory-file-system.cpp

// Time:  ls: O(l + klogk), l is the path length, k is the number of entries in the last level directory
//        mkdir: O(l)
//        addContentToFile: O(l + c), c is the content size
//        readContentFromFile: O(l + c)
// Space: O(n + s), n is the number of dir/file nodes, s is the total content size.

class FileSystem {
public:
    FileSystem() : root_(new TrieNode()) {
        
    }
    
    vector<string> ls(string path) {
        auto curr = getNode(path);
        
        if (curr->isFile) {
            return {split(path, '/').back()};
        }
        
        vector<string> result;
        for (const auto& child : curr->children) {
            result.emplace_back(child.first);
        }
        sort(result.begin(), result.end());
        return result; 
    }
    
    void mkdir(string path) {
        auto curr = putNode(path);
        curr->isFile = false;
    }
    
    void addContentToFile(string filePath, string content) {
        auto curr = putNode(filePath);
        curr->isFile = true;
        curr->content += content;
    }
    
    string readContentFromFile(string filePath) {
        return getNode(filePath)->content;
    }

private:
    struct TrieNode {
        bool isFile;
        unordered_map<string, TrieNode *> children;
        string content;
    };

    TrieNode *root_;
    
    TrieNode *getNode(const string& path) {
        vector<string> strs = split(path, '/');
        auto curr = root_;
        for (const auto& str : strs) {
            curr = curr->children[str];
        }
        return curr;
    }

    TrieNode *putNode(const string& path) {
        vector<string> strs = split(path, '/');
        auto curr = root_;
        for (const auto& str : strs) {
            if (!curr->children.count(str)) {
                curr->children[str] = new TrieNode();
            }
            curr = curr->children[str];
        }
        return curr;
    }

    vector<string> split(const string& s, const char delim) {
        vector<string> tokens;
        stringstream ss(s);
        string token;
        while (getline(ss, token, delim)) {
            if (!token.empty()) {
                tokens.emplace_back(token);
            }
        }
        return tokens;
    }
};

/**
 * Your FileSystem object will be instantiated and called as such:
 * FileSystem obj = new FileSystem();
 * vector<string> param_1 = obj.ls(path);
 * obj.mkdir(path);
 * obj.addContentToFile(filePath,content);
 * string param_4 = obj.readContentFromFile(filePath);
 */