476. Number Complement（补数）

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.

3. 思路：给定一个正整数，输出它的补数。补数的策略是通过翻转二进位表示。
4. 1.求出该数的二进制位数； 
   2.通过给定数num和相同位数的二进制数（全为1）进行异或，即可求出补数。

5.  public int findComplement(int num) {
               
               int ans = 0;//the count of the num's bits
               int temp = num;//copy of the num
               while(temp != 0){
                   ans++;
                   temp /= 2;
               }
               return num ^ (int)(Math.pow(2,ans)-1); 
       }

   pow() 函数用来求 x 的 y 次幂（次方），其原型为：
            double pow(double x, double y);