[bit manipulation] 476. Number Complement

Given a positive integer, output its complement number. The complement strategy is to flip the bits of its binary representation.

Note:
The given integer is guaranteed to fit within the range of a 32-bit signed integer.
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.

~num取反 但是把所有leading 0都跟着取反了。例如5: 101, ~5 不是010, 是1111..010。
所以制作一个mask,111, 使~num & mask即可。
如何制作mask?一个可以到处用的trick:

(Integer.highestOneBit(num) << 1) – 1

例如5,Integer.highestOneBit(num) = 0b100, 右移一位变成 1000,再减1变成111

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