# [476. Number Complement](https://leetcode.com/problems/number-complement/)


## 题目

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.


## 题目大意

给定一个正整数，输出它的补数。补数是对该数的二进制表示取反。

注意:

给定的整数保证在32位带符号整数的范围内。
你可以假定二进制数不包含前导零位。



## 解题思路


- 求一个正数的补数，补数的定义是对该数的二进制表示取反。当前不能改变符号位。按照题意构造相应的 mask 再取反即可。



## 代码

```go

package leetcode

// 解法一
func findComplement(num int) int {
	xx := ^0 // ^0 = 1111111111111111111111
	for xx&num > 0 {
		xx <<= 1 // 构造出来的 xx = 1111111…000000，0 的个数就是 num 的长度
	}
	return ^xx ^ num // xx ^ num，结果是前面的 0 全是 1 的num，再取反即是答案
}

// 解法二
func findComplement1(num int) int {
	temp := 1
	for temp <= num {
		temp <<= 1 // 构造出来的 temp = 00000……10000，末尾 0 的个数是 num 的长度
	}
	return (temp - 1) ^ num // temp - 1 即是前面都是 0，num 长度的末尾都是 1 的数，再异或 num 即是最终结果
}

```