[LeetCode] 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"

 将十进制数转换成十六进制数，并将十六进制数转换成字符串。因为十六进制数每一位由二进制数每4位组成。所以将十进制数用二进制表示，然后每4位转换成十六进制数，[num & 0xf]就是取二进制数的最低4位。因为-1的二进制每一位都是1，所以引入count计数，只计算32位数即可。

class Solution {
public:
    const string HEX = "0123456789abcdef";
    string toHex(int num) {
        string s;
        if (num == 0)
            return "0";
        int count = 0;
        while (num && count++ < 8) {
            s = HEX[num & 0xf] + s;
            num >>= 4;
        }
        return s;
    }
};
// 3 ms