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.

簡單來說就是要他的補數拉
本來我想的非常簡單
就是直接
return ~num就好

但是後來才發現因為他是用int 32bit表示
所以假設你是題目的範例5 => 101的話 他的補數會變成 前面多出29個1再來才是010
最後就是-6...

因此邏輯上需要將前面的0通通遮起來
作法如下

class Solution {
public:
    int findComplement(int num) 
    {  
        unsigned int mask = ~0;
        //根據num的bit數調整mask  
        while(num&mask) mask <<= 1;  
        return ~mask & ~num;  
    }  
};