十進(jìn)制轉(zhuǎn)十六進(jìn)制。
除了0，首位都不為0；字母全部小寫；num可能為負(fù)數(shù)。
題目：405. Convert a Number to Hexadecimal

另外二進(jìn)制位運(yùn)算加法 題目：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"

二進(jìn)制，計(jì)算每四位得到一個十六進(jìn)制的位。

需要注意，因?yàn)榭赡転樨?fù)數(shù)，所以用>>>：

位運(yùn)算符
“>>” 右移，高位補(bǔ)符號位，右移1位表示除2 （若值為正，則在高位插入0；若值為負(fù)，則在高位插入1）
“>>>” 無符號右移，高位補(bǔ)0（Java獨(dú)有，無論正負(fù)，都在高位插入0）
“<<” 左移，左移1為表示乘2

負(fù)數(shù)的補(bǔ)碼表示：
正數(shù)的補(bǔ)碼與原碼相同，負(fù)數(shù)的補(bǔ)碼為對該數(shù)的原碼除符號位外各位取反，然后在最后一位加1.
-5 在計(jì)算機(jī)中表達(dá)為：11111111 11111111 11111111 11111011。轉(zhuǎn)換為十六進(jìn)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;
    }
}