【leetcode】476. Number Complement

　　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 num, return its complement.

Example 1:

Input: num = 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.

          这题考察对于数字基本的位存储的信息，依次取出数字的最低位，如果是0则翻转相加，index记录当前的阶数，num补零右移，直到为0位置。

class Solution {
public:
    int findComplement(int num) {
        int index=0;
        int tmp=0;
        while(num){
            if((num&1)==0){
                tmp=tmp+pow(2,index);
            }
            num=num>>1;
            index++;
        }
        return tmp;
    }
};

      还有一种更加trick的处理方式。利用bit mask的方式计算

class Solution {
public:
    int findComplement(int num) {
        unsigned mask = ~0;
        while( mask & num ) mask = mask << 1;
        return ~num ^ mask;
    }
};