URL字符解码

将网页编码文字还原：

例如：https%3A%2F%2Fwww.example.com%2F%3Fparam%3Dvalue%26key%3D%E4%B8%AD%E6%96%87

解码：

https: // www.example.com/?param=value&key=中文

代码：

char hexValue(char ch)
{if (isdigit(ch)){return ch - '0';}return ::toupper(ch) - 'A' + 10;
}std::string urlDecode(std::string& input)
{std::string decoded;size_t inputLen = input.size();for (size_t i = 0; i < inputLen; i++){char c = input[i];if (c == '+'){decoded.push_back(' ');}else if (c == '%' && i + 2 < inputLen){char hex1 = input[++i];char hex2 = input[++i];if (isxdigit(hex1) && isxdigit(hex2)){char decodedChar = (hexValue(hex1) << 4) | hexValue(hex2);decoded.push_back(decodedChar);}else{decoded.push_back(c);decoded.push_back(hex1);decoded.push_back(hex2);}}else{decoded.push_back(c);}}return decoded;
}