405. 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"

解法一：Language-Java Run Time-7ms
注意：一個數以二進制形式存放在內存中，負數以它的補碼形式存在

    public String toHex(int num) {
        StringBuffer sb = new StringBuffer();
        if(num == 0)
        {
            sb.append(0);
        }else{
            while(num != 0){
                //取num的後位
                int n = num & 0xf;
                if(n >= 0xa){
                    n = n - 10 + 'a';
                }else{
                    n = n + '0';
                }
                sb.append((char)n);
                //無符號右移，忽略符號位，空位都以0補齊
                num = num >>> 4;
            }
        }
        return sb.reverse().toString();
    }