476. Number Complement

Description

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.

Solution

Bit manipulation

也是有妙招啊。把num除去开头的0的其余位全变成1也是挺厉害的。

class Solution {
    public int findComplement(int num) {
        int mask = (highestOneBit(num) << 1) - 1;
        return num ^ mask;
    }
    
    public int highestOneBit(int n) {
        int res = 0x80000000;
        while ((n & res) == 0) {
            res >>>= 1; // important to use unsigned bit shift
        }
        return res;
    }
}