第一种：比较陈旧

//URLEncode
+ (NSString*) encodeString:(NSString*)unencodedString {
    
    // CharactersToBeEscaped = @":/?&=;+!@#$()~',*";
    // CharactersToLeaveUnescaped = @"[].";
    
    NSString *encodedString = (NSString *)
    CFBridgingRelease(CFURLCreateStringByAddingPercentEscapes(kCFAllocatorDefault,
                                                              (CFStringRef)unencodedString,
                                                              NULL,
                                                              (CFStringRef)@"!*'();:@&=+$,/?%#[]",
                                                              kCFStringEncodingUTF8));
    
    return encodedString;
}

//URLDEcode
-(NSString *)decodeString:(NSString*)encodedString

{
    //NSString *decodedString = [encodedString stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding ];

    NSString *decodedString  = (__bridge_transfer NSString *)CFURLCreateStringByReplacingPercentEscapesUsingEncoding(NULL,
                                                                                                                     (__bridge CFStringRef)encodedString,
                                                                                                                     CFSTR(""),
                                                                                                                     CFStringConvertNSStringEncodingToEncoding(NSUTF8StringEncoding));
    return decodedString;
}

第二种：ios7.0之后

//encode
[string stringByAddingPercentEncodingWithAllowedCharacters:[NSCharacterSet URLQueryAllowedCharacterSet]];
//decode
[string stringByRemovingPercentEncoding];

第三种：自定义

NSCharacterSet`系统内置的一些返回CharacterSet 的方法
- URLFragmentAllowedCharacterSet  "#%<>[\]^`{|}
- URLHostAllowedCharacterSet      "#%/<>?@\^`{|}
- URLPasswordAllowedCharacterSet  "#%/:<>?@[\]^`{|}
- URLPathAllowedCharacterSet      "#%;<>?[\]^`{|}
- URLQueryAllowedCharacterSet     "#%<>[\]^`{|}
- URLUserAllowedCharacterSet      "#%/:<>?@[\]^`

NSCharacterSet *chaSet = [NSCharacterSet characterSetWithCharactersInString:@"'();:@&=+$,/?%#[]"]; 
[string stringByAddingPercentEncodingWithAllowedCharacters:chaSet];

- URLFragmentAllowedCharacterSet  "#%<>[\]^`{|}
- URLHostAllowedCharacterSet      "#%/<>?@\^`{|}
- URLPasswordAllowedCharacterSet  "#%/:<>?@[\]^`{|}
- URLPathAllowedCharacterSet      "#%;<>?[\]^`{|}
- URLQueryAllowedCharacterSet     "#%<>[\]^`{|}
- URLUserAllowedCharacterSet      "#%/:<>?@[\]^`


这些的意思是这些字符需要按照URL的规则被以%开头的编码替换，如果某个字符不想被替换，则不要写在""之间即可。