Not getting the approach

#1

https://codeforces.com/problemset/problem/535/B

please help me in this question, i am not getting the approach, i read the editorial also but didn’t understand anything

#2

I’ll explain how I solved it.

If you write down the numbers in order,

4, 7, 44, 47, 74, 77, 444, 447, ...

As there are only 2 numbers, we try to replace them by 1 and 0 and see that they look like binary numbers

0, 1, 00, 01, 10, 11, 000, 001, ...

But this makes no sense, as (0 and 00 and 000 … are all same), similarly (1 and 01 and 001 …).

We find that zero is spoiling it, so we use 1 and 2 instead of 0 and 1.

1, 2, 11, 12, 21, 22, 111, 112, ...

position of 1 = 1 * 1 = 1
position of 2 = 2 * 1 = 2
position of 11 = 1 * 1 + 1 * 2 = 3
position of 12 = 2 * 1 + 1 * 2 = 4
position of 21 = 1 * 1 + 2 * 2 = 5
position of 22 = 2 * 1 + 2 * 2 = 6
position of 111 = 1 * 1 + 1 * 2 + 1 * 4 = 7
position of 112 = 2 * 1 + 1 * 2 + 1 * 4 = 8

Now you can easily see that they are exactly equal to their positions in base 2.

Therefore the code can be just,

    int n;
    cin >> n;
    int ans = 0;
    int ctr = 1;
    int val;
    while (n)
    {
        int d = n % 10;
        n /= 10;
        if (d == 4)
            val = 1;
        else
            val = 2;
        ans += (ctr * val);
        ctr <<= 1;
    }
    cout << ans;
#3

what is meant by position on rank 2… and in code what is the meaning of cin >> n; and ctr <<= 1;

#4

I don’t get your doubt.

‘cin >> n’ is input in C++.

ctr <<= 1; is the left shift (<<) operator in C++.

ctr <<= 1;
// same as ctr *= 2;