发现NSString类中有内置的方法可以实现。他们分别是：
- (NSString *)stringByAddingPercentEscapesUsingEncoding:(NSStringEncoding)encoding
- (NSString *)stringByReplacingPercentEscapesUsingEncoding:(NSStringEncoding)encoding
只要传入相应的编码即可以进行编码和解码了，不过此方法是对整个Url进行编码的所以如果有Query String中带有&?/等是不会进行编码转换的，如果有这方面的需要的朋友可以参照下面提供的网址，找到解决方法：http://www.cocoachina.com/bbs/read.php?tid-2469.html
示例：
NSString* string1 = @"https://www.cloudsafe.com/文件夹";
NSString* string2 = [string1 stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSString* string3 = [string2 stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSString* string4 = [string2 stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSString* string5 = [string3 stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSString* string6 = [string4 stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSString* string7 = [string5 stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
输出结果: string5 & 6 重复编码encode
string1:https://www.cloudsafe.com/文件夹
string2:https://www.cloudsafe.com/%E6%96%87%E4%BB%B6%E5%A4%B9
string3:https://www.cloudsafe.com/%25E6%2596%2587%25E4%25BB%25B6%25E5%25A4%25B9
string4:https://www.cloudsafe.com/文件夹
string5:https://www.cloudsafe.com/%E6%96%87%E4%BB%B6%E5%A4%B9
string6:https://www.cloudsafe.com/文件夹
string7:https://www.cloudsafe.com/文件夹