Unlucky Numbers

There are few numbers considered to be unlucky(It contains only 4 and 7. ). Our goal is to find count of such numbers in the range of positive integers a and b.
For Example:
Input:: a = 10 b = 20
Output:: 0

Input:: a = 30 b = 50
Output:: 2 (44, 47)

Source: Topcoder

Solution:
Idea is to find the number of digits in a, let that be n. now
1. Start with a number which has n 4s, so if n is 2 start with 44, if n i 5 start with 44444.
2. increment the count.
3. If the curr number greater than b break and print the count.
4. Now start with the curr digit(last digit) of curr number, if the curr is 4 change it to 7 and
go to step2. if it is 7 set the curr digit to 4 and go to next digit to the left. do this step 4 again
until u reach a number greater than b, or reach a place where all digits are 7s and then u add 4
to it and go to step2.


Example:
a = 10 b = 500
actual numbers are 44, 47, 74, 77, 444, 447, 474, 477