Convert a Number to Hexadecimal

Problem

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

Note:

All letters in hexadecimal (a-f) must be in lowercase. 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. The given number is guaranteed to fit within the range of a 32-bit signed integer. 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"

Solution

Bit Manipulation Solution

public class Solution {
    public String toHex(int num) {
        if (num == 0) return "0";
        StringBuilder sb = new StringBuilder();
        char[] digits = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
        while (num != 0) { //Cannot use num > 0 because of negative numbers 
            sb.append(digits[num & 15]);
            num = num >>> 4;
        }
        return sb.reverse().toString();
    }
}

Analysis

This is a bit manipulation solution
We use num & 15 to get index in hexadecimal number
Then we shift the number 4 bits because 2 ^ 4 = 16
This solution covers the negative numbers, which is awesome
Complexity:

• Time: O(n), linear time used
• Space: O(n), where n is the digits of hexadecimal number