# [1009. Complement of Base 10 Integer](https://leetcode.com/problems/complement-of-base-10-integer/)


## 题目

The **complement** of an integer is the integer you get when you flip all the `0`'s to `1`'s and all the `1`'s to `0`'s in its binary representation.

- For example, The integer `5` is `"101"` in binary and its **complement** is `"010"` which is the integer `2`.

Given an integer `n`, return *its complement*.

**Example 1:**

```
Input: n = 5
Output: 2
Explanation: 5 is "101" in binary, with complement "010" in binary, which is 2 in base-10.

```

**Example 2:**

```
Input: n = 7
Output: 0
Explanation: 7 is "111" in binary, with complement "000" in binary, which is 0 in base-10.

```

**Example 3:**

```
Input: n = 10
Output: 5
Explanation: 10 is "1010" in binary, with complement "0101" in binary, which is 5 in base-10.

```

**Constraints:**

- `0 <= n < 109`

## 题目大意

每个非负整数 N 都有其二进制表示。例如， 5 可以被表示为二进制 "101"，11 可以用二进制 "1011" 表示，依此类推。注意，除 N = 0 外，任何二进制表示中都不含前导零。

二进制的反码表示是将每个 1 改为 0 且每个 0 变为 1。例如，二进制数 "101" 的二进制反码为 "010"。

给你一个十进制数 N，请你返回其二进制表示的反码所对应的十进制整数。

## 解题思路

- 简单题。求一个十进制数的反码，只需要让该数和全 1 的数进行异或计算即可。所以本题重点在如何构造 mask 上。

## 代码

```go
package leetcode

func bitwiseComplement(n int) int {
    mask := 1 
    for mask < n {
        mask = (mask << 1) + 1
    }
    return mask ^ n
}
```