[leetcode #476] Number Complement

Problem

The complement of an integer is the integer you get when you flip all the 0's to 1's and all the 1's to 0's in its binary representation.

For example, The integer 5 is "101" in binary and its complement is "010" which is the integer 2.
Given an integer num, return its complement.

Example 1:

Input: num = 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: num = 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.

Constraints:

1 <= num < 2³¹

Note: This question is the same as 1009: https://leetcode.com/problems/complement-of-base-10-integer/

Idea

주어진 수의 complement를 구하는 문제다.

Bit Mask를 만든 뒤 XOR 연산을 해도 되지만, 더 간단한 방법이 있다.
주어진 수보다 큰 2의 제곱 중 최소값을 구한 뒤, 그 값에서 주어진 수를 빼면 된다. Complement의 원리를 생각하면 XOR 연산 없이 금방 풀 수 있음을 알 수 있다.

Solution

class Solution {
    public int findComplement(int num) {
        int n = 0;
        while (n < num) {
            n = (n << 1) | 1;
        }
        return n - num;
    }
}

Reference

https://leetcode.com/problems/number-complement/