Power of 2

• Time : $O(|A|logA)$
• Space : $O(|A|)$

int Solution::power(string A) {
    if(A == "1") return 0;
    while((A.back() - '0') % 2 == 0) {
        string B = "";
        int carry = A[0] == '1' ? 1 : 0;
        for(int i = A[0] == '1' ? 1 : 0; i < A.length(); i++) {
            int now = carry * 10 + A[i] - '0';
            char ch = now / 2 + '0';
            B.push_back(ch);
            carry = now % 2;
        }
        swap(A,B);
    }
    return A == "1";
}