1009-complement-of-base-10-integer.py

"""
Problem Link: https://leetcode.com/problems/complement-of-base-10-integer/

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 n, return its complement.

Example 1:
Input: n = 5
Output: 2
Explanation: 5 is "101" in binary, with complement "010" in binary, which is 2 in base-10.

Example 2:
Input: n = 7
Output: 0
Explanation: 7 is "111" in binary, with complement "000" in binary, which is 0 in base-10.

Example 3:
Input: n = 10
Output: 5
Explanation: 10 is "1010" in binary, with complement "0101" in binary, which is 5 in base-10.
 
Constraints:
0 <= n < 109
"""
# Time Complexity: O(logN)
class Solution:
    def bitwiseComplement(self, n: int) -> int:
        mask = 1
        
        """
        (number) => (how-number-is-derived)=>binary-string
        mask = 1 => 1 => 1
        mask = 3 => (2*1 + 1) => 11
        mask = 7 => (3*2 + 1) => 111
        mask = 15 => (7*2 + 1) => 1111
        """
        while mask < n:
            mask = mask * 2 + 1
        
        return mask ^ n