LeetCode 405. Convert a Number to Hexadecimal

问题描述：

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.

思路：问题很简单，把10进制的数转化为16进制的字符串，正数的思路十分简单，负数比较复杂，一开始想的是把负数num转为正数2^32+num，那么他的编码就是该负数的编码，但是2^32超过了上界，想了很久解决不了负数的问题，最后看solution，非常优雅：

class Solution {public:string toHex(int num) {const string map = "0123456789abcdef";if (num == 0)return "0";string res = "";int c = 0;while (num && c++ <8) {res = map[num & 15] + res;cout << num << endl;num >>= 4;}return res;}};