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:

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:
26

Output:
"1a"

Example 2:

Input:
-1

Output:
"ffffffff"


>>> hex(123)
'0x7b'
>>> hex(123)[2:]
'7b'
>>> hex(1)
'0x1'
>>> hex(0)
'0x0'
>>> bin(0)
'0b0'
>>> bin(1)
'0b1'

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class Solution(object):     def toHex(self, num):         """         :type num: int         :rtype: str         """         # use recurively                if num < 0:             return hex(num+(1<<32))[2:]         else:             return hex(num)[2:]

 注：Main ideal is to flip the negative number to positive by using following code: # num = num + 2**32

负数的binary 表示就是num + 2**32的正数表示。因为python里hex和bin都是针对正数有效。

>>> bin(-1)
'-0b1'
>>> bin(-12)
'-0b1100'
>>> hex(-12)
'-0xc'
>>> hex(-123)
'-0x7b'

因此，自己写的话，可以：

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class Solution(object):     def toHex(self, num):         """         :type num: int         :rtype: str         """         # use recurively                if num == 0: return "0"                    if num < 0:             num += (1<<32)         ans = ""         hex_s = "0123456789abcdef"         while num != 0:             ans = hex_s[num & 0xf] + ans             num = num >> 4         return ans

 其他解法：

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def toHex(self, num):         if num==0: return '0'         mp = '0123456789abcdef'  # like a map         ans = ''         for i in range(8):             n = num & 15       # this means num & 1111b             c = mp[n]          # get the hex char             ans = c + ans             num = num >> 4         return ans.lstrip('0')  #strip leading zeroes