InMemoryFileSystem.swift

class FileSystem {
    class File {
        var isFile = false
        var files: [String: File] = [String: File]()
        var content = ""
    }
    var root: File!
    init() {
        root = File()
    }
    
    func ls(_ path: String) -> [String] {
        var temp: File = root
        var files: [String] = []
        if path != "/" {
            let d: [String] = path.components(separatedBy: "/")
            for i in 0..<d.count {
                if let file = temp.files[d[i]] {
                   temp = file
                }
            }
            if temp.isFile, let last = d.last {
                files.append(last)
                return files
            }
        }
        var result = temp.files.keys.sorted()
        return result
    }
    
    func mkdir(_ path: String) {
        var temp: File? = root
        var d = path.split(separator: "/")
        for i in 0..<d.count {
            if temp?.files[String(d[i])] == nil {
                temp?.files[String(d[i])] = File()
            }
            temp = temp?.files[String(d[i])]
        }
    }
    
    func addContentToFile(_ filePath: String, _ content: String) {
         var temp: File? = root
        var d = filePath.split(separator: "/")
        for i in 0..<d.count - 1 {
            temp = temp?.files[String(d[i])]
        }
        if temp?.files[String(d[d.count - 1])] == nil {
            temp?.files[String(d[d.count - 1])] = File()
        }
        temp = temp?.files[String(d[d.count - 1])]
        temp?.isFile = true
        temp?.content = temp!.content + content
    }
    
    func readContentFromFile(_ filePath: String) -> String {
        var temp: File = root
        var d = filePath.split(separator: "/")
        for i in 0..<(d.count - 1) {
            if let file = temp.files[String(d[i])] {
               temp = file
            }
        }
        if let last = d.last, let file: File = temp.files[String(last)] {
            return file.content
        }
        return ""
    }
}
/**
 * Your FileSystem object will be instantiated and called as such:
 * let obj = FileSystem()
 * let ret_1: [String] = obj.ls(path)
 * obj.mkdir(path)
 * obj.addContentToFile(filePath, content)
 * let ret_4: String = obj.readContentFromFile(filePath)
 */