Given an input 'n' from the user, you need to find all lucky numbers from 1 to n. Lucky numbers are those which have either 4 or 7 or both as their digits. What is the most efficient method to solve this problem?

Theoretically, you could create a compile time table if upper bound of n is known. Each item of the table holds lucky numbers seen till then. Index in to the table with input number.

The other solution can be: Step 1- count the number of digits of 'n'. Say it is 'd'. Step 2- now for i=1 to d, generate the numbers from 0 to (2^i) -1 in binary and replace 0 with 4 and 1 with 7. Also if(i>=1 and i<= d-1) then print the numbers. and when i=d, check whether the number is smaller than n, then print it else ignore.

Let X be any lucky number, then you can generate another two lucky numbers from X. i.e. "10*X+4" and "10*X+7"..!! You can do it anyway, but here is the recursive solution..!! I hope this will help... Link To The Solution: http://paste.ubuntu.com/7632294/ (Solution by kakajan on codeforces) Codeforces Blog: http://212.193.33.25/blog/entry/12659