題目：

Given a positive integer, output its complement number. The complement strategy is to flip the bits of its binary representation. Note: The given integer is guaranteed to fit within the range of a 32-bit signed integer. 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.

解釋： 二進位制取反。 python投機取巧: 先用bin變成二進位制，然後用1減去每一位進行翻轉 c++: 如果我們能知道該數最高位的1所在的位置，就可以構造一個長度和該資料所佔位置一樣長的一個掩碼mask，然後該數字和mask進行異或即可。 例如：5的二進位制是101，我們的構造的掩碼為mask=111，兩者異或則為010，即是所要的結果。 取反就是和與它長度相同的全1二進位制進行異或

python程式碼：

class Solution(object):
    def findComplement(self, num):
        """
        :type num: int
        :rtype: int
        """
        num_bin=bin(num)[2:]
        result=''
        for n in num_bin:
            r=str(1-int(n))
            result+=r
        return int(result,2)
 

c++程式碼：

class Solution {
public:
    int findComplement(int num) {
        int mask=1;
        int n=num;
        while(n>0)
        {
            mask=mask<<1;
            n=n>>1;
        }
        return (mask-1)^num;
    }
};

python寫法：

class Solution(object):
    def findComplement(self, num):
        """
        :type num: int
        :rtype: int
        """
        n=num
        mask=1
        while(n>0):
            mask=mask<<1
            n=n>>1
        return (mask-1)^num
 

這種異或的寫法速度顯然更快。

總結： 取反就是和與它長度相同的全1二進位制進行異或