LeetCode 405. Convert a Number to Hexadecimal （进制转换，移位运算）

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:26Output:"1a"

Example 2:

Input:-1Output:"ffffffff"

输入一个整数，输出其十六进制表示的字符串。负数使用补码表示。

思路：用十进制的15（二进制的1111）作为掩码，获取最低的4位，直接查表映射成十六进制。然后右移4位，进行下次映射。

在右移位时，注意要转换成无符号整数，这样移位时才能补0，否则将添加符号位。

    string toHex(int num) {        if(num==0)return "0";        string ans = "";        string hexChar = "0123456789abcdef";        while (num) {            ans = hexChar[num & 15] + ans;            num = (unsigned)num >> 4;        }        return ans;    }