476. Number Complement (solution 2)

package LeetCode_476

/**
 * 476. Number Complement
 * https://leetcode.com/problems/number-complement/description/
 *
Given a positive integer num, output its complement number.The complement strategy is to flip the bits of its binary representation.

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:
The given integer num is guaranteed to fit within the range of a 32-bit signed integer.
num >= 1
You could assume no leading zero bit in the integer’s binary representation.
 * */
class Solution {
    /*
    solution 2: Decimal to binary, flip the bits then cover it to Decimal, Time:O(logn), Space:O(logn)
     * */
    fun bitwiseComplement(N: Int): Int {
        //solution 2
        val list = ArrayList<Int>()
        var num = N
        //decimal to binary
        while (num > 0) {
            list.add(num % 2)
            num = num shr 1
        }
        //flip the binary
        for (i in list.indices) {
            if (list.get(i) == 1) {
                list.set(i, 0)
            } else {
                list.set(i, 1)
            }
        }
        var result = 0
        var count = list.size - 1.0
        //turn into decimal, binary to decimal by math formula: (base two) 101 = 1*2^2 + 0*2^1 + 1*2^0
        while (count >= 0) {
            result += list.get(count.toInt()) * Math.pow(2.0, count).toInt()
            count--
        }
        return result
    }
}