示例 1：
​
输入:
26
​
输出:
"1a"
示例 2：
​
输入:
-1
​
输出:
"ffffffff"

class Solution {
char u16[16]={'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f'};    
int temp=15;
public:
    string toHex(int num) {
        if(num==0) return "0";
        unsigned int N=num;
        string res;      
        while(N)
        {
            //cout<<num<<endl;
            int index=(N & temp);
            res=u16[index]+res;
            N>>=4;
        }
        return res;
​
    }
};