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:

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  (no leading zero bits), and its complement is . 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。

巴特西

位运算（6）——Number Complement

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