Hello, Can anyone explain idea to solve E. Lucky Array problem. I tried to understand from editorial but I'm finding difficult to understand. Problem Link:E. Lucky Array
Problem Description: Petya loves lucky numbers. Everybody knows that lucky numbers are positive integers whose decimal representation contains only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.
Petya has an array consisting of n numbers. He wants to perform m operations of two types:
add l r d — add an integer d to all elements whose indexes belong to the interval from l to r, inclusive (1 ≤ l ≤ r ≤ n, 1 ≤ d ≤ 10^4);
count l r — find and print on the screen how many lucky numbers there are among elements with indexes that belong to the interval from l to r inclusive (1 ≤ l ≤ r ≤ n).
Each lucky number should be counted as many times as it appears in the interval. Petya has a list of all operations. The operations are such that after all additions the array won't have numbers that would exceed 10^4. Help Petya write a program that would perform these operations.