[LeetCode]--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:

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:
-1

Output:
"ffffffff"

把一个十进制数转换成用字符串表示的16进制字符串，不需要过多的考虑负数问题，因为负数在做位操作时已经自动转换成了补码+1的形式直接进行正常操作即可
每四位取一次，用一个数组表示对应关系

public String toHex(int num) {
        if (num >= 0 && num < 10)
            return Integer.toString(num);
        char[] hex = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a',
                'b', 'c', 'd', 'e', 'f' };
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 8 && num != 0; i++) {
            sb.insert(0, hex[num & 15]);
            num = num >> 4;
        }
        return sb.toString();
    }