Problem
Submission
I think the issue arises from this block

else{
        if(remaining_elements%2==0){
            cout<<(x+remaining_elements)<<"\n";
        }else{
            if(on_bits<=1){
                if(remaining_elements==1){
                    if(x==1){
                        cout<<(x+4)<<"\n";
                    }else{
                        cout<<(x+n)<<"\n";
                    }
                }else{
                    cout<<( x+n+3)<<"\n";
                }
            }else{
                cout<<(x+remaining_elements+1)<<"\n";
            }
        }
    }

remaining_elements = n-(number of on bits of x)
here if the on_bits = 1 or on_bits = 0(i.e x = 0 and ive already handled x = 0 case), so the only way control of the program reaches here would be if x = 1 or if $$$x = 2^k$$$
if x = 1 and remaining_elements = 1, then

$$$A = [a, b]$$$, here answer is $$$x+4$$$
else x!=1 that means $$$x = 2^k$$$ where $$$k \gt 0$$$, here answer is $$$x+n$$$, reasoning:

here $$$ans = x + 2$$$ or $$$ans = x + n$$$(since remaining_elements = 1 and on_bits = 1, n=2)
in the else block answer is $$$x+n+3$$$, reasoning
else assumes that on_bits>1(2,3,4,...) and remaining_elements can be {1,3,...} if