十六进制字符串转换为二进制字符串，二进制字符串转换为十进制字符串

看某华为面试题目有感，十六进制转换为二进制很简单，同理八进制，四进制转换为二进制亦很简单。

参考网上代码，二进制转换为十进制亦很简单，不过这想法真的是很巧妙，学习到了。

//十六进制转换为二进制
string HexToBin(string& str)
{
    string res;
    size_t i, len = str.length();
    if (len <= 0) return res;

    for (i = 0; i < len; i++)
    {
        if (str[i] == '0')
            res += "0000";
        else if (str[i] == '1')
            res += "0001";
        else if (str[i] == '1')
            res += "0001";
        else if (str[i] == '2')
            res += "0010";
        else if (str[i] == '3')
            res += "0011";
        else if (str[i] == '4')
            res += "0100";
        else if (str[i] == '5')
            res += "0101";
        else if (str[i] == '6')
            res += "0110";
        else if (str[i] == '7')
            res += "0111";
        else if (str[i] == '8')
            res += "1000";
        else if (str[i] == '9')
            res += "1001";
        else if (str[i] == 'a')
            res += "1010";
        else if (str[i] == 'b')
            res += "1011";
        else if (str[i] == 'c')
            res += "1100";
        else if (str[i] == 'd')
            res += "1101";
        else if (str[i] == 'e')
            res += "1110";
        else if (str[i] == 'f')
            res += "1111";
        else {
            cout << "--ERROR--" << endl; break;
        }
    }
    return res;
}

//进制转换中间变量
struct TBigInt
{
    int len;
    char value[10000];
    TBigInt() :len(0)
    {
        for (size_t i = 0; i < 10000; i++)
            value[i] = 0;
    }
};

void AddBit(TBigInt& data, char BitVal)
{
    size_t i = 0, len = data.len;
    for (i = 0; i < len; i++)
    {
        data.value[i] *= 2;
    }
    data.value[0] += BitVal;
    for (i = 0; i < len; i++)
    {
        data.value[i + 1] += data.value[i] / 10;
        data.value[i] %= 10;
    }
    if (data.value[data.len])
        ++data.len;
}

string OutputIntStr(TBigInt& data)
{
    int i;
    string res;
    if (0 == data.len)
    {
        res.push_back('\0');
    }
    else
    {
        for (i = data.len - 1; i >= 0; --i)
        {
            res.push_back(data.value[i] + '0');
        }
    }
    return res;
}

//二进制字符串转换为十进制字符串
string Bin2Deci(const string& str)
{
    size_t i;
    TBigInt data;
    for (i = 0; i < str.length(); i++)
    {
        AddBit(data, str[i] == '1' ? 1 : 0);
    }
    return OutputIntStr(data);
}

int main()
{
    int pos;
    string str, res;
    getline(cin, str);
    res = HexToBin(str);
    for (size_t i = 0; i < res.length(); i++)
    {
        if (res[i] != '0')
        {
            pos = i; break;
        }
    }
    res = res.substr(pos);
    cout << str << "->" << res << endl;
    cout << res << "->" << Bin2Deci(res) << endl;
    system("pause");
    return 0;
}