A strong number is a special type of number whose sum of the factorial of digits is equal to the original number. In other words, if we take the digits of a number, find the factorial of each digit, and add them together, the resulting sum is equal to the original number. For example, 145 is a strong number because 1! + 4! + 5! = 1 + 24 + 120 = 145.

To check if a number is a strong number, we can follow these steps:

1. Take each digit starting from the unit place and find its factorial.
2. Add the factorials of each digit.
3. Compare the result with the original number. If they are equal, then the number is a strong number; otherwise, it is not.

We can also print all strong numbers less than or equal to a given number N by iterating from 1 to N and checking if each number is a strong number or not.