题目及理解

题目链接: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 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.

理解

水题,就是按位取反

代码

class Solution {
    public:    
        int findComplement(int num) {        
            int flag = 0;        
            int ans=0,a=1;        
            while (num > 0){            
                flag = num % 2;            
                num /= 2;            
                ans += (flag==0?1:0) * a;            
                a*=2;        
            }        
        return ans;    
    }
};

其他解法

看了Discuss里面的解法,还是有很多骚操作的大神的.这里贴出一个来看看.
链接

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

理解

先让mask全部取1,然后对num取&操作,使得在对应前导0的地方全部置1,剩下的位全部是0,然后再对2个数进行&操作,就是取反了.