題目如下：

Given a number N, return a string consisting of "0"s and "1"s that represents its value in base -2 (negative two).

The returned string must have no leading zeroes, unless the string is "0".

Example 1:

Input: 2
Output: "110"
Explantion: (-2) ^ 2 + (-2) ^ 1 = 2

Example 2:

Input: 3
Output: "111"
Explantion: (-2) ^ 2 + (-2) ^ 1 + (-2) ^ 0 = 3
 

Example 3:

Input: 4
Output: "100"
Explantion: (-2) ^ 2 = 4

Note:

1. 0 <= N <= 10^9

解題思路：以12為例，轉成二進制後是1100，假設最右邊的下標為0，從右往左下標一次加1。很顯然，下標是偶數位的情況，值是正數；而下標為奇數位的情況值是負數。為了要彌補負數帶來的影響，需要加上等於這個負數*2的絕對值，而這個值恰好就是該負數的下一位。因此只要奇數位出現了1，那麽相應就要在其後面的偶數位加上1，如果偶數位本來就是1，則需要考慮進位。

代碼如下：

class Solution(object):
    
 
def baseNeg2(self, N):
        """
        :type N: int
        :rtype: str
        """
        bs = bin(N)[2:][::-1]
        res =  [int(i) for i in list(bs)]

        def carrier(res,i):
            if i == len(res) - 1:
                res += [1]
                if len(res) % 2 == 0:
                    res 
 
+= [1]
            else:
                res[i + 1] += 1
        for i in range(1,len(res)):
            if res[i] == 0:
                continue
            elif res[i] == 1 and i % 2 == 1:
                carrier(res,i)
            elif res[i] == 2:
                res[i] = 0
                carrier(res, i)
        return ‘‘.join([str(i) for i in res[::-1]])

【leetcode】1017. Convert to Base -2