*****"将字典保存城市数组的例子练习一遍"*****

    1),将字典写入文件中
        - (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile;
        - (BOOL)writeToURL:(NSURL *)url atomically:(BOOL)atomically;



    2),从文件中读取字典
        + (id)dictionaryWithContentsOfFile:(NSString *)path;
        + (id)dictionaryWithContentsOfURL:(NSURL *)url;

*****"将字典保存城市数组的例子练习一遍"*****

    1),将字典写入文件中
        - (BOOL)writeToFile:(NSString *)path atomically:(BOOL)useAuxiliaryFile;
        - (BOOL)writeToURL:(NSURL *)url atomically:(BOOL)atomically;



    2),从文件中读取字典
        + (id)dictionaryWithContentsOfFile:(NSString *)path;
        + (id)dictionaryWithContentsOfURL:(NSURL *)url;