[NSString rangeOfString:].location返回一个NSUInteger,如果找不到这个子字符串,NSRange返回(NSNotFound,0),NSNotFound.location返回NSNotFound,即2147483647,即2^31-1。
An NSRange structure giving the location and length in the receiver of the first occurrence of aString. Returns {NSNotFound, 0} if aString is not found or is empty (@"").