NSString *str = @"www.baidu.com";
NSURL *URL = [NSURL URLWithString:str]; //string>url
NSString *str1 = [URL absoluteString]; //url>string
NSString *str = @"www.baidu.com";
NSURL *URL = [NSURL URLWithString:str]; //string>url
NSString *str1 = [URL absoluteString]; //url>string