405. Convert a Number to Hexadecimal (Easy)

https://leetcode.com/problems/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"

Solutions

class Solution {

    Map<Integer, String> map;

    private void init() {
        if (map != null) {
            return;
        }

        map = new HashMap<>();
        map.put(0, "0");
        map.put(1, "1");
        map.put(2, "2");
        map.put(3, "3");
        map.put(4, "4");
        map.put(5, "5");
        map.put(6, "6");
        map.put(7, "7");
        map.put(8, "8");
        map.put(9, "9");
        map.put(10, "a");
        map.put(11, "b");
        map.put(12, "c");
        map.put(13, "d");
        map.put(14, "e");
        map.put(15, "f");
    }

    public String toHex(int num) {
        init();

        if (num == 0) {
            return "0";
        }

        long numx = num;
        if (num < 0) {
            // Note that 2l must be long type and int doesn't work.
            numx = (2l << 31) + num;
        }

        String ans = "";
        while (numx != 0) {
            long remainder = numx % 16;

            ans = map.get((int) remainder) + ans;

            numx = numx / 16;
        }

        return ans;
    }
}

Incorrect Solutions

References