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:

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Input:
26
Output:
"1a"

Example 2:

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Input:
-1
Output:
"ffffffff"

题目还是比较简单的，就是要处理负数比较麻烦。

  1. 这里用 num & 15 代替了 num % 16. 无论num正负，都可以。
  2. num >>> 4 代替了 num / 16. >>> 跟 >> 的区别是 >> 会按照符号进行补位，如果num是负数，就会补1, 而>>>不管正负都补0。
  3. 另外，负数是用补码(Two’s complement)表示的。譬如说一个负数为-a, -a和a的二进制之间的关系为 -a = ~a + 1。
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public String toHex(int num) {
    if (num == 0)
        return "0";
    char[] chars = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
    StringBuilder sb = new StringBuilder();
    while (num != 0) {
        sb.insert(0, chars[num & 15]);
        num = num >>> 4;
    }
    return sb.toString();
}