LeetCode_476. Number Complement

476. Number Complement

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 guanteed 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 compilement 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 compilement is 0. So you need to output 0.

题目理解：
给定一个正整数，输出其补数。求补策略是二进制位反转。
注意：给定整数确定在32位带符号整数内；假设整数的二进制不包含前导0。

思路：
一般来说，正数的补码是它本身，负数的补码是取反加1（符号位不变），根据题意的取反策略，这里所说的complement应该指的是反码。用最大数减去该数就能得到其反码。
备注：
反码–one’s complement(对1求补)
补码–two’s complement(对2求补)

C++:

class Solution{
public:
    int findComplement(int num){
    int quot=num, m=1;
    while(quot>0)
    {
        m=m*2;
        quot=quot/2;
    }
    return m-1-num;
    }
};

3line C++:
https://discuss.leetcode.com/topic/74627/3-line-c/3

// init mask=1111 1111;
// for example,
// num = 0000 0101
// mask = 1111 1000
// ~mask & ~num = 0000 0010
class Solution{
public:
    int findComplement(int num){
        unsigned mask = ~0;  //mask=1111 1111
        while (num & mask)  mask<<=1;
        return ~mask & ~num;
    }
};