[位运算]405. Convert a Number to Hexadecimal

十进制转十六进制。
除了0，首位都不为0；字母全部小写；num可能为负数。
题目：405. Convert a Number to Hexadecimal

另外二进制位运算加法 题目：371. Sum of Two Integers

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:-1Output:"ffffffff"

二进制，计算每四位得到一个十六进制的位。

需要注意，因为可能为负数，所以用>>>：

位运算符
“>>” 右移，高位补符号位，右移1位表示除2 （若值为正，则在高位插入0；若值为负，则在高位插入1）
“>>>” 无符号右移，高位补0（Java独有，无论正负，都在高位插入0）
“<<” 左移，左移1为表示乘2

负数的补码表示：
正数的补码与原码相同，负数的补码为对该数的原码除符号位外各位取反，然后在最后一位加1.
-5 在计算机中表达为：11111111 11111111 11111111 11111011。转换为十六进制：0xFFFFFFFB。

Runtime: 9 ms

class Solution {
    public String toHex(int num) {
        if(num == 0) return "0";
        char[] Hex = {'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f'};
        String res = "";
        while(num != 0){
            res = Hex[num & 15] + res; //15 is a mask : '1111'
            num = num >>> 4;
        }
        return res;
    }
}