LeetCode 405. Convert a Number to Hexadecimal

原题链接在这里：https://leetcode.com/problems/convert-a-number-to-hexadecimal/

题目：

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"

题解：

每4位组成一个hexadecimal digit, 每次取出最后4位和1111做位运算&得到一个digit.

num 右移4位，这里用>>>而不是>>. e.g. -1的two’s complement表示为ffffffff, 32位全是1 bit. 若用>> 首位的1 bit不会移动, while loop的condition num!=0不会出现.

Time Complexity: O(1). Space: O(1).

AC Java:

public class Solution {
    public String toHex(int num) {
        if(num == 0){
            return "0";
        }
        
        StringBuilder sb = new StringBuilder();
        while(num != 0){
            int digit = num & 0xf;
            sb.insert(0, digit<10 ? (char)(digit+'0') : (char)(digit-10+'a'));
            num >>>= 4;
        }
        return sb.toString();
    }
}