476. Number Complement

题目描述：

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.

解题思路：
这题一种比较好的解法是使用bitset类，但平时没用过，类的具体方法不记得，所以使用笨方法。
首先计算得到输入整数的位数n，通过观察可以发现要得到的结果是2^n - 1 - 输入整数的值。

代码：

class Solution {
public:
    int findComplement(int num) {
        int length = 0, tmp = num;
        while (num != 0) {
            num /= 2;
            length++;
        }
        return pow(2, length) - 1 - tmp;
    }
};