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: 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.

---

思路：

num          = 00000101
mask         = 11111000
~mask & ~num = 00000010

代码1：

class Solution {
public:
    int findComplement(int num) {
        bool start = false;
        for (int i = 31; i >= 0; --i) {
            if (num & (1 << i)) start = true;
            if (start) num ^= (1 << i);
        }
        return num;
        
    }
};

代码2：

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