Suppose the given number is N, the result is R, it's easy to notice that

N + R + 1 = 2^x, and x is unknown, but it's quite easy to calculate.

for x from 0 to 31, the first x which makes 2^x -1 >= N is the target.

but N = 0 is an exception, just take care of it in the beginning.

    class Solution {
        public int bitwiseComplement(int N) {
            if (N == 0) return 1;
            long num = 1;
            while (num - 1 < N) {
                num *= 2;
            }
            return (int)(num - 1 - N);
        }
    }