Matthew leetcode-405.Convert a Number to Hexadecimal

class Solution {
public:

    string toHex(int num) {
        string ret;
        if (num == 0) ret = "0";
        else if (num > 0) {
            ret = toHex1(num);
        }
        else {
            // 得到一个 1 0000 0000 0000 0000 0000 0000 0000 0000的数
            unsigned long long n = (unsigned long long ) pow(2,32) - 1;
            // n就变成了无符号位的，二进制后32位与原来的负数一样的数
            n = n & num;
            ret = toHex1(num);
        }
        return ret.size() > 8?ret.substr(ret.size() - 8) : ret;
    }
    string toHex1(unsigned long long num) {
        string ret;
        string hex[] = {"0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f"};
        while (num) {
            int pos = num % 16;
            num /= 16;
            ret += hex[pos];
        }
        reverse(ret.begin(), ret.end());
        return ret;
    }
};