python--leetcode476. Number Complement

Given a positive integer, output its complement number. The complement strategy is to flip the bits of its binary representation.

Note:

1. The given integer is guaranteed to fit within the range of a 32-bit signed integer.
2. You could assume no leading zero bit in the integer’s binary representation.

Example 1:

Input: 5
Output: 2
Explanation: The binary representation of 5 is 101 (no leading zero bits), and its complement is 010. So you need to output 2.

Example 2:

Input: 1
Output: 0
Explanation: The binary representation of 1 is 1 (no leading zero bits), and its complement is 0. So you need to output 0.

这一题的意思就是给你一个在int范围内的数，让你求它的补码。当然请注意此题的补码不是你计组里学的那个补码，所以想直接求反加一是不行的。

此题两种解法，第一种自然就是暴力解:

class Solution(object):
    def findComplement(self, num):
        binary = bin(num)[2:]
        ans = 0
        for i, u in enumerate(binary[::-1]):
            if u == '0':
                ans += 2 ** i
        return ans

由于使用bin函数转十进制为二进制，得到的是一个0b开头的数字，所以取[2:]得到纯数字，用[::-1]翻转列表。

更快捷的解法为位运算：

class Solution(object):
    def findComplement(self, num):
        i = 1
        while i <= num:
            i = i << 1
        return (i - 1) ^ num