leetcode 405 Convert a Number to Hexadecimal
z
 2019-05-25 11:00:43  

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"
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class Solution(object):
    def toHex(self, num):
        """
        :type num: int
        :rtype: str
        """
        dic = {10: 'a', 11: 'b', 12: 'c', 13: 'd', 14: 'e', 15: 'f'}
        
        if num == 0:
            return '0'
        
        if num  < 0:
            num = num + 2**32
        
        ans = []
        
        while num != 0:
            t = num % 16
            num = num / 16
            if t < 10:
                ans.insert(0, str(t))
            else:
                ans.insert(0, dic[t])
        return ''.join(ans);

KeyNodes:

  • 负数的情况比较复杂，可以考虑直接从负数转换为对应的正数的编码。

    • 方法一：

      直接在负数的基础上加上$2^{32}$

    • 方法二：

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      num = -num
      num ^= 0xffffffff
      num += 1

  • 注意保存在ans之后，需要进行reverse操作

  • 如果使用str来保存最后的结果，可以使用str[::-1]来进行reverse操作。