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.

我的解答：（找到第一個1，算法過於臃腫）

class Solution {
public:
    int findComplement(int num) {
        bool meat1 = false;
        int flag = false;
        for(int i = 8 * sizeof(int) - 1; i >= 0 ; i--){
            if(!meat1){
                flag = num & (0x1 << i);
            }
 
            if(flag){
                meat1 = true;
            }
            
            if(meat1){
                num ^= (0x1 << i);    
            }
        }
        return num;
    }
};

Leetcode上看到的很快速的解法

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

For example,

num          = 00000101
mask         = 11111000
~mask & ~num = 00000010

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