476. 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: 5Output: 2Explanation: 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: 1Output: 0Explanation: The binary representation of 1 is 1 (no leading zero bits), and its complement is 0. So you need to output 0.

解析： 发现与位数相同的全1求异或能得到答案，下面怎么求得位数相同的全1，发现不好求，先求全0再取反，求全0的方法是对全1和原数相与并左移直到得到0为止

代码：

class Solution {public:    int findComplement(int num) {        int ans=0;        unsigned int mask=~0;        while (num&mask) mask<<=1;        mask=~mask;        return num^mask;    }};