leetcode405+十进制转换为16进制，位运算

https://leetcode.com/problems/convert-a-number-to-hexadecimal/description/

class Solution {
public:
    string toHex(int num) {
        string res = "";
        for(int i=0; num&&i<8; i++){
            int t = num& 0xf;
            if(t>=10) res = char('a'+t-10) + res;
            else res = char('0'+t) + res;
            num >>= 4;
        }
        return res.empty()?"0":res;
    }
};