## Problem

Given a positive integer, output its complement number. The complement strategy is to flip the bits of its 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.
```

**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.
- This question is the same as 1009: https://leetcode.com/problems/complement-of-base-10-integer/

## Solution

### Bitwising

Flip every bit from right to left

```
class Solution {
func findComplement(_ num: Int) -> Int {
if num == 0 { return 1 }
var it = num
var res = 0
var mask = 1
while it > 0 {
if it & 1 == 0 {
res |= mask
}
mask <<= 1
it >>= 1
}
return res
}
}
```

### Substraction

complement + num + 1 = 2^n (n is bit length of num)

example: 101 + 010 + 1 = 1000

```
class Solution {
func findComplement(_ num: Int) -> Int {
var higher = 1
while higher <= num {
higher <<= 1
}
return higher - num - 1
}
}
```