Convert a Number to Hexadecimal

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.

Example 1:

Input:
26

Output:
"1a"

Example 2:

Input:
-1

Output:
"ffffffff"

Normally we will think of do divide and mod for the problem. However, for all decimals that are 2^n, we can just divide the number by groups of 2^n bits, in this case, 4 bit, and get the answer. This solution is applied to all 2^n decimals.

public String toHex(int num) {
        if(num == 0) return "0";
        char[] map = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
        StringBuilder builder = new StringBuilder();
        while (num != 0) {
            builder.append(map[num & 15]);
            num = (num >>> 4);
        }
        return builder.reverse().toString();
        
    }