Directi interview experience | Set 25 (Off campus for Platform Engineer)

Method of applying : I applied to Directi via employee referral.

Round 1: Online coding round (Codechef , 90 minutes)

Problem 1 : Easy, Adhoc

Statement

You are given a string of numeric digits, you have to find smallest number possible using these digits with no leading zeroes.

Example
If input is 330101 then answer is 100133

Constraint

Length of string <= 10^5

Problem 2 :

Statement

You are given two strings, say A and B, of the same length N. You can swap A[i] and B[i] for all i between 1 and N, inclusive. You cannot swap two characters within A, or within B. Also, you can only swap a character in A with the character at the same index in B, and with no other character. You can perform this operation zero or more times.

You wish to modify the strings through the operations in such a way that the number of unique characters in the strings is small. In fact if n(A) is the number of unique characters in A and n(B) is the number of unique characters in B; you wish to perform the operations such that max(n(A),n(B)) is as small as possible.

Print the value of max(n(A),n(B)) after all the operations.

Example

If input is
ababa babab

Then output should be one because we can swap and make the pair of strings
(aaaaa , bbbbb)

Constraints

1 <= T <= 100
1 <= length(A) <= 16
length(B) = length(A)

Problem 3 : 

Modified Knapsack DP problem

Round 2: Algorithm round (Skype , 45 – 50 minutes)

A square grid(NxN) is given to you; Each location on the grid is either a brick (B) or its empty (_).

The total number of bricks is exactly equal to as much is required to build a “wall” in the grid. See example for clearer understanding.

That is, at the end , all bricks(B) should be placed at boundary locations.

For moving a brick from location <x,y> to <i,j> |i-x| + |j-y| fuel is used.

Each brick in the grid can be moved to any location on the boundary with equal probability. What is the expected value of the fuel required to do so? Each brick can be moved at-most once.

Example
In the end (after moving all the bricks), the grid should look like:

 B B B B
 B _ _ B
 B _ _ B
 B B B B

Hint
You are given the initial locations of all the bricks , and you know the final positions.
Since all bricks are the same, you can put any brick in any boundary positions.

Lets say there are ‘b’ boundary locations = b bricks
So you need to map the old locations to new ones.

In brute force approach , it would be O(b!) to try out all placings.
But you can do better with a little more insight of how the probabilities add up brickwise instead of arrangement wise.

Expected : O(n)
Round 3: Algorithm Round (Skype , 45 – 50 minutes )

Round 4: Technical knowledge round (Skype , 45 minutes )

• Talk about a project of yours in depth. Be ready to answer any question regarding it. You may also talk about your internship project. I talked about my GSoC project in my case.
• Be ready to answer anything that’s on your resume. I was asked to talk about another project listed on my resume.
• How would you implement the diff functionality of git ? Be ready for follow up questions and further optimizations on the spot.
• I was given a sample code , with one add_balance() and one subtract_balance() function, and asked to explain the problem if two threads access it. How would you rectify it?(Using mutex lock)
  • Follow up question : How (do you think) are mutex lock operations wait and signal implemented in OS?
• Database Indexing
  • What are indexes? How are they useful?
  • If you had to build an index, what data structure would you use?
  • Why Btree and not normal BSTs (when the number of comparisons is actually greater for Btrees)?