C++ URL encode/decode (gb2312)

    char Dec2HexChar(short int n)
    {
        if ( 0 <= n && n <= 9 ) {
            return char( short('0') + n );
        } else if ( 10 <= n && n <= 15 ) {
            return char( short('A') + n - 10 );
        } else {
            return char(0);
        }
    }  

    short int HexChar2Dec(char c)
    {
        if ( '0'<=c && c<='9' ) {
            return short(c-'0');
        } else if ( 'a'<=c && c<='f' ) {
            return ( short(c-'a') + 10 );
        } else if ( 'A'<=c && c<='F' ) {
            return ( short(c-'A') + 10 );
        } else {
            return -1;
        }
    }  

    string EncodeURL(const string &URL)
    {
        string strResult = "";
        for ( unsigned int i=0; i<URL.size(); i++ )
        {
            char c = URL[i];
            if (
                ( '0'<=c && c<='9' ) ||
                ( 'a'<=c && c<='z' ) ||
                ( 'A'<=c && c<='Z' ) ||
                c=='/' || c=='.'
                ) {
                    strResult += c;
            }
            else
            {
                int j = (short int)c;
                if ( j < 0 )
                {
                    j += 256;
                }
                int i1, i0;
                i1 = j / 16;
                i0 = j - i1*16;
                strResult += '%';
                strResult += Dec2HexChar(i1);
                strResult += Dec2HexChar(i0);
            }
        }  

        return strResult;
    }  

    std::string DecodeURL(const std::string &URL)
    {
        string result = "";
        for ( unsigned int i=0; i<URL.size(); i++ )
        {
            char c = URL[i];
            if ( c != '%' )
            {
                result += c;
            }
            else
            {
                char c1 = URL[++i];
                char c0 = URL[++i];
                int num = 0;
                num += HexChar2Dec(c1) * 16 + HexChar2Dec(c0);
                result += char(num);
            }
        }  

        return result;
    }