PayPal Full Stack Engineer Interview Questions and Answers

Interview preparation tips for other job seekers - "GfG must do" should be solved properly alteast , otherwise go keep on practicing on any platform .

Round: Technical Interview
Tips: Have a good grasp of DS algo, java, etc.(Here CGPA doesn't matter)

General Tips: Do’s and Don’ts :
1. Be confident and to be confident prepare well.
2. Set the priority of your companies 
3. Don't start preparation at end. Don't ignore the content of resume.

Be confident and start preparation as soon as possible.
College Name: IIT Kanpur

Round: Test
Experience: Online Test on mettl.com
- 2 coding questions ( 15*2 = 30 marks ) - Moderate level
- 10 MCQs on Programming and Java ( 1*10 = 10)
- 6 MCQs on Machine Learning and Hadoop ( 1*6 = 6 )
- 6 MCQs on Computer Networks ( 1*6 = 6 )
- 6 MCQs on Computer Infrastructure and Architecture ( 1*6 = 6)
- 6 MCQs on Application security and Cryptography ( 1*6 = 6)
15 shortlisted from here including 8 B.Techs and 7 M.Techs for F2F interviews, which were taken by VISA employees from their US and Singapore offices( since they don't have any office at India ).

Round: Technical Interview
Experience: They may give you a situation and ask you to explain which Software engineering principles will you use. They ask you how will you explain the concept of stacks and queues to a 7 year old, or to a granny, so as to encourage programming(like seriously ? )

General Tips: Nothing as such, since this was my second on-campus placement, I had good experience of interviews.
Keep your fundamentals in all computer science subjects open. Unlike other companies they don't focus much on coding alone. The only other company which also looks at perfection in all computer science aspects is Xerox Research Center India.
Challenging and Equally rewarding, but seriously a unique experience with no much tension
Skills:
College Name: NIT Surathkal

Round: Test
Experience: The test was on hackerrank.
Tips: * Fast coding and problem solving required
* Time crunch
* The test is valued a LOT in the hiring process
Duration: 120 minutes
Total Questions: 4

Round: Technical Interview
Experience: Question 1 : He tried to take the discussion to web based technologies to which I straightly told I don’t know. Not sure if that was a good decision. Nevertheless, he went over my resume a lot of times and asked about various projects.
Question 2 : Discussed the various projects related to it.
Question 3 : He asked this question because I had worked on django during my FB internship. I proposed a tree based structure. xyz.com/a/b/c ---> b is child of a, c is child of b.

Question 4 : Next he moved to a very interesting question. Given a simple text editor, I want to facilitate autocomplete feature. We discussed about syntactic methods using language specific keywords. I suggested a trie based implementation which he liked. Later he asked me to improve and redirected me to using scope of the variables, and grammar of the language. We discussed about all the steps I did in language translators lab (macrojava -> minijava -> typechecking -> miniIR -> microIR -> miniRA ->Mips) where we built a Java compiler in Java. We later hopped back to original question and I suggested that we can use grammar to to store multiple (nested kind of) tries for different scope. He asked me how would I define “scope”. He gave me a scenario which helped me come up with the nested trie structure
int a;
{
int b;
{
int c; //a b c can be used
}
{
int d; //a b d can be used
}
}
He then asked me details of how I would code. I said we could use the respective visitors and maintain the current trie root for scope. He liked it but I missed at 2 places :
1. He asked me about token generation used in a compiler and I said flex and bison. I was expected to elaborate a bit.
2. He asked me how the grammar is generated and how the visitors are invoked. I couldn’t explain properly because I was not thorough with it.
Tips: * Keep smiling in the interviews.
* My interviewer had hopped from Microsoft to Visa. We had a short detailed discussion on how the transition was and what drove him to do that.
* It’s always good to know about the company to pick up interesting discussions, read up latest articles and news
* The interviewers want to see if you are passionate about what they are building. Their PPT (Pre Placement Talk) was also focussed majorly on the technologies they are building.
* If you take a question in a direction, make sure you are very very thorough with it. I took it in the direction of compilers, but couldn't make the transition seamlessly because I wasn't thorough with a lot of things (like token generation, visitor invocation).
* Not sure if telling upfront that you don't know about web technologies when you are applying for a profile that might need it is a good idea or not.


Round: Technical Interview
Experience: I think he was a senior person and spoke very passionately about the work that is happening in visa Bangalore office.

Question 1 : I said maybe I should have learnt a little about web based technologies. I think he was looking for a more generic answer.
Question 2 : I went ahead with calculating the expected score where my score random variable = 1 with probability p, and 0 with probability (1-p). I conditioned that expected score of game1 > game2 which would give me the range of p. I goofed up the equations twice but he said he liked my approach and I was just there

Question 3 : I suggested building a graph where a node is a word and an edge means a transition, and then performing BFS. He said that was obvious and that he was more interested in knowing how I build the graph. I suggested a method to reduce the number of string comparisons by storing a count vector with each word. Count vector is a 26 length vector where count[i] = number of times that letter occurs in the word. I would do string comparison to identify neighbours only if the difference in count vectors in 2. Eg, cot = cat - a + o. String comparison is still needed because of false positives. Eg, oct = cat - a + o but cat -> oct is not a valid transition. He liked my approach and asked me to code, discussed the complexity.
Tips: * Don't be naive around a senior engineer. Be confident.
* Explain your approach before going ahead solving it. I missed that in Q1 and for a long time he didn't know what I was heading towards.
* Write clean code, practice coding on paper to make sure you write compilable code. Shabby code is a turn off.
* Show passion for the company you are applying for.

General Tips: * Be friendly and keep smiling.
* Know about the company, well in detail. Recent news, articles, acquisitions, organisational changes, new offices, new product, etc.
* Know about the volume the company handles, certain mind boggling figures, what the company *does not* do
* If you are introducing yourself as an expert in a certain field, be prepared for questions like "How will you contribute to our company with your expertise" type of questions.
* Be very very thorough with every single thing you write on your resume. A small point you weren't sure about might become the basis of your whole interview. Don't write things you are unsure of, or have worked very little on.
Skill Tips: * I think I didn’t take Visa very seriously. I was assuming they won’t take a undergrad and they won’t take me because I have 0 knowledge in web based technologies. I might have acted a little cocky.
* They had nice pie diagrams which shows that they had done thorough research about each and every candidate based on resume and the coding test.
* They value the coding test a lot.
* Be very thorough with the projects you write on resume. They will give you a problem which won't be straightforward and will map onto one of your course projects. You are expected to do the mapping seamlessly. Every bit of the discussion you have with the interviewer can be your next problem.
* Brush up on web based technologies if you can.
* There will be a HR round as well if you clear the technical interviews.

Skills: Maths(esp Probability), Design, Passion, Web Technologies, Technical Skill And Speaking Skill, Coding Skills
College Name: IIT Madras
Motivation: * Been the leader since more than half a century, been through the technological revolution.
* Revolutionising payments architecture. Building something of greatest need, so elegant and optimised, yet producing such consistent performance.
* Keeping up with the need of the generation by introducing features which are brand new. Eg, “all-purpose credit card”.
* Loved their PPT (Pre Placement Talk). Volume of the data and processing is IMMENSE. Billions of transactions in a year voluming up to trillions. More than 200 countries in the world --> these stats are HUGE.
* Fraud detection is a space I would have loved to work on given that I come from a Data Mining background.

Funny Moments: Chucked out before HR round :P

Round: Test
Experience: Coding Questions were fairly standard ones.
Duration: 75 minutes
Total Questions: 42

Round: Interview
Experience: Networks based questions, firewalls, TCP/IP and other OSI layers

Round: Interview
Experience: Coding questions. Asked to code on paper. Questions on Object Oriented Design.

Skills: Networks basics, OOPs basics, Coding Skills
College Name: IIT Guwahati
Motivation: Profiles offered, Package, Location