[LeetCode] 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.

求一个数的补码，就是对一个数的二进制每一位求反。即对每一位与1求XOR。这需要知道原数字的二进制数的位数，这样才可以得到需要的1的位数，按位取XOR即可。

class Solution {
public:
    int findComplement(int num) {
        int tmp = num, mask = 1;
        while (tmp != 0) {
            tmp >>= 1;
            mask <<= 1;
        }
        return (mask - 1) ^ num;
    }
};
// 3 ms