Consider a series of numbers composed of only digits 4 and 7. First few numbers in the series are 4, 7, 44, 47, 74, 77, 444, .. etc. Given a number constructed by 4, 7 digit only, we need to find position of this number in this series.
Input : 7 Output : pos = 2 Input : 444 Output : pos = 7
It is reverse of the following article :
Find n-th element in a series with only 2 digits (4 and 7) allowed | Set 2 (log(n) method)
"" / \ 1(4) 2(7) / \ / \ 3(44) 4(47) 5(74) 6(77) / \ / \ / \ / \
The idea is based on the fact that all even positioned numbers have 7 as last digit and all odd positioned numbers have 4 as last digit.
If number is 4 then it is the left node of the tree then it corresponds to (pos+2)+1. Else right child node(7) corresponds to (pos*2)+2.
This article is contributed by Devanshu Agarwal. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to email@example.com. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
- Position of n among the numbers made of 2, 3, 5 & 7
- Find the n-th number made of even digits only
- Find the number of occurrences of a character upto preceding position
- N-th character in the string made by concatenating natural numbers
- Check if elements of array can be made equal by multiplying given prime numbers
- Nth number made up of odd digits only
- Find the longest string that can be made up of other strings from the array
- Largest number divisible by 90 that can be made using 0 and 5
- Total number of different staircase that can made from N boxes
- Finding n-th number made of prime digits (2, 3, 5 and 7) only
- Count number of strings (made of R, G and B) using given combination
- Check whether the number can be made perfect square after adding 1
- Find if given number is sum of first n natural numbers
- Find the starting indices of the substrings in string (S) which is made by concatenating all words from a list(L)
- Find the number of consecutive zero at the end after multiplying n numbers
Improved By : Sam007