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: 

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Input:
26
Output:
"1a"

Example 2: 

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Input:
-1
Output:
"ffffffff"

思路

我们利用位运算来解决这个问题。所有的数在内存中都是以二进制的方式存储的。二进制转十六进制的方法非常简单，只要每四位二进制数转换成一位十六进制数即可。

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import math
class Solution(object):
    def toHex(self, num):
        """
        :type num: int
        :rtype: str
        """
        
        hexmap = ['0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f']
        reslist = []
        mask = 0xf
        flag = 0
        iter = 0
        # 注意小于0的情况
        if (num < 0):
            num = num & 0x7fffffff
            iter = 7
            flag = 1
        elif(num == 0):
            return '0'
        else:
            iter = int(math.log(abs(num), 16))
            
        reslist.append( hexmap[num & mask] )
        for i in range(iter):
            num = num >> 4
            tmp = num & mask
            if (i == iter-1 and flag == 1):
                tmp += 8
            reslist.append( hexmap[tmp] )
        reslist.reverse()
        res = ''.join(reslist)
        return res