Leetcode405. Convert a Number to Hexadecimal十进制转十六进制

Given an integer, write an algorithm to convert it to hexadecimal. For negative integer, two’s complement method is used.

Note:

1. All letters in hexadecimal (a-f) must be in lowercase.
2. The hexadecimal string must not contain extra leading 0s. If the number is zero, it is represented by a single zero character '0'; otherwise, the first character in the hexadecimal string will not be the zero character.
3. The given number is guaranteed to fit within the range of a 32-bit signed integer.
4. You must not use any method provided by the library which converts/formats the number to hex directly.

class Solution {
public:
    char* hex = "0123456789abcdef";
    string toHex(int num) {
        return toHexEx(num);
    }
    
    string toHexEx(unsigned int num) {
        string s;
        if (num > 15) {
            unsigned int sub = num % 16;
            s += hex[sub];
            return toHexEx(num >> 4) + s;
        } else {
            s += hex[num];
            return s;
        }
    }
};