May Leetcode Challenge - Day 4

Problem

Number Complement

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

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

Solution

Bitwising

Flip every bit from right to left

class Solution {
    func findComplement(_ num: Int) -> Int {
        if num == 0 { return 1 }
        var it = num
        var res = 0
        var mask = 1
        
        while it > 0 {
            if it & 1 == 0 {
                res |= mask
            }
            
            mask <<= 1
            it >>= 1
        }
        
        return res
    }
}

Substraction

complement + num + 1 = 2^n (n is bit length of num)
example: 101 + 010 + 1 = 1000

class Solution {
    func findComplement(_ num: Int) -> Int {
        var higher = 1 
        while higher <= num {
            higher <<= 1
        }
        return higher - num - 1
    }
}