Convert a Number to Hexadecimal

题目：

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"

代码：

class Solution {public:    string toHex(int num) {        if(num==0) return "0";        string ans="";        string table[16]={"0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f"};        string temp="";        unsigned int tempnum=(unsigned) num;        while(tempnum!=0)        {            ans=table[tempnum&15]+ans;            tempnum>>=4;        }        return ans;                    }};

分析：

      十进制转十六进制每四位为单位进行移位运算，无符号右移为每移动移位高位补0，有符号右移是正数补0，负数补1，所以对负数右移时首先将负数转化为无符号数，再进行右移。