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 

0

s. 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"

不用管这个数字十进制是什么，用二进制转换为十六进制。

注意负数的情况右移位是要用符号位‘1’来补的，所以永远都不会到0，于是用一个cnt来记录移位次数（int 32bit，最多移次）

class Solution {
public:
string toHex(int num) {
string hex = "0123456789abcdef";
string ans = "";
int cnt = 0;
while (num != 0 && cnt++ < 8) {
ans.push_back(hex[num & 15]);
num >>= 4;
}
reverse(ans.begin(), ans.end());
if (ans.empty()) {
ans = "0";
}
return ans;
}
};