405. 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 librarywhich converts/formats the number to hex directly.

Example 1:

Input:
26

Output:
"1a"

Example 2:

Input:
-1

Output:
"ffffffff"

Basic idea: each time look at the last four digits of binary verion of the input, and maps that to a hex char shift the input to the right by 4 bits, keep doing it until num == 0.

time: O(1), space: O(1)

class Solution {
    public String toHex(int num) {
        if(num == 0) {
            return "0";
        }
        char[] base = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
        StringBuilder sb = new StringBuilder();
        boolean isLeading = true;
        for(int i = 28; i >= 0; i -= 4) {
            int digit = (num >> i) & 15;
            if(digit != 0 || !isLeading) {
                sb.append(base[digit]);
                isLeading = false;
            }
        }
        return sb.toString();
    }
}