位运算（6）——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.

public class Solution {
    public int findComplement(int num) {
        int mask = (Integer.highestOneBit(num) << 1) - 1;
        num = ~num;
        return num & mask;
    }
}

Integer.highestOneBit(num); //01101返回1000，001返回1，101返回100

为什么需要mask：

例如num = 0000 1010，那么 ~num = 1111 0101，加入mask = 0000 1111后，~num前面的1都消除了。