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Can you solve this interview problem?

Shuvo on November 04, 2021

So few days ago I applied to a MNC and I had some interesting problems to solve in given amount of time. So let me share one with you, see if you c...

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GrahamTheDev • Nov 4 '21 • Edited

Yes I can solve the interview problem (as in, the problem with interviews in the industry in general!).

I would look at the person interviewing me dead in the eyes and ask "who hurt them". What made them think in any way this question remotely resembles something you would need to do in the real world?

I would look at them asking why on Earth they thought it was a good interview question, what skills it proved I had that I would need to learn and just generally ask them why we can't create an interview process that actually proves talent.

I would ask them what happened to our industry where they thought this question is remotely useful and then would slowly rise from my seat, thank them for the opportunity and walk to the door.

Just as the door is about to close I would say "And please can I have the 🍬 treat 🍬 you promised me for solving it" and walk away, still searching for a company that doesn't ask dumb interview questions. 😉🤣

Being Serious for a second, besides the dig at our industry and it's stupid interview practices it was a fun little brain teaser. Have a ❤🦄 for a nice morning wake up challenge!

Keff • Nov 4 '21

Yeah, I would run away from there as fast as possible. Can't imagine what the interviewer could do, seems like a psychopath really.

Just kidding, but I agree, this is not the type of question we should be asking in an interview, it shows nothing of value for the real world. I think this is more of a practice exercise for begginers more than anything. Though it's a fun little problem

Shuvo • Nov 4 '21

Actually it was really interesting
Because the like you'd finally decode would lead you to the next step of recruitment

GrahamTheDev • Nov 4 '21 • Edited

Haha now this would make it more interesting as remote interview technique, you can only proceed to the next step if you can decode the link.

You could also make it time sensitive so if they take too long to complete the task the link expires!

Still hate questions like this (yet again, not a stab at you, just our industry), but that line of thinking, a "follow the breadcrumbs" type of coding interview, that is an awesome and fun idea 🤣

Keff • Nov 4 '21

That's really cool, gamifing it and making it somewhat of a puzzle. I guess the complexity would increment as you solved more steps right?

Keff • Nov 4 '21

Yup, that's quite cool actually, making the process a bit more fun.

brunnerh • Nov 4 '21

If nothing is known about the URL beforehand, this seems underspecified. There are multiple valid URLs that can be constructed from the fragments.

function permutate(arr) {
    if (arr.length == 1) return [arr];

    return arr.map((item, i) => {
        var rest = [...arr];
        rest.splice(i, 1);
        return permutate(rest).map(x => [item, ...x])
    }).flat();
}

permutate(JSON.parse(url))
  .map(x => atob(x.join('')))
  .map(x => { try { return new URL(x).toString() } catch { return null } })
  .filter(Boolean);

Yields:

https://dev.xn--tocommen%7C1414%20-oqa05ipa4v8fnd624dn60pha/1je1m
https://dev.tocomment/1je1m%C3%B2%C3%B3%076%C2%87Wf%C3%B3
https://dev.to/0shuvo0/comment/1je1m
https://dev.to/0shuvo0-%0B%C3%8CZ%C2%99L[%7D%C2%8D%C2%BD%C2%B5%C2%B5%C2%95%C2%B9

Shuvo • Nov 4 '21

You can take those multiple URLs as a challenge as well
Like sending https request to see the status

brunnerh • Nov 4 '21

Those are additional assumption about the URL, though:

It uses a common web protocol like HTTP/HTTPS
There actually is a reachable server that would reply to those requests

Rushan S J • Nov 6 '21

Hey, I got the solution. The only thing is that I'm more familiar with python but the same logic applied in other languages too.

import itertools, base64

originalarray = ["6Ly9kZXYudG", "9jb21tZW5", "8vMHNodXZvMC", "aHR0cHM", "0LzFqZTFt"]     # this is the input 

for i in itertools.permutations(originalarray, len(originalarray)):  # getting all possible combinations
    tmpdecodedstring = ""
    for k in i:     # iterate over each strng in the array and add it to the string
        tmpdecodedstring += str(k)
    try:
        print("URL is: " + str(str(base64.b64decode(tmpdecodedstring)).decode()))  # decode the string finally
    except:
        pass # only proper base64 string dont raise an error. If there is a error, that means that the string is incorrect and we try another permutation.

Shuvo • Nov 6 '21

Great

Keff • Nov 4 '21

Some people just want to see the world burn lol!!!!

Fun exercise nonetheless, here is what I came up with in 5 minutes. I did it in NodeJS though.

A couple of notes on the solution:

It only works with simple urls as the one mentioned in the example, it wont work if the URI has query params, or more complex structure
I had to google how to do permutations as I have never needed to use it in the real world
If we wanted to check for more complex urls, we could update the isValidUrl function to do so, without changing anything else

But basically the logic is this:

Convert string into an array by removing unnecessary characters ([]") and then split by comma (,)
Permutate the array
Filter all non-valid urls
What you're left with is the correct URL

const permutator = (inputArr) => {
    let result = [];

    const permute = (arr, m = []) => {
        if (arr.length === 0) {
            result.push(m);
        } else {
            for (let i = 0; i < arr.length; i++) {
                const curr = arr.slice();
                const next = curr.splice(i, 1);
                permute(curr.slice(), m.concat(next));
            }
        }
    }

    permute(inputArr);

    return result;
}

const isValidUrl = (candidateUrl) => {
    return /^https:\/\/dev.to\/([\w0-9]+\/?)+$/i.test(candidateUrl);
}

const array = url.replace(/[\[ \]"]/g, '').split(',')
const correctUrl = permutator(array)
    .map(p => p.join(''))
    .map(s => Buffer.from(s, 'base64').toString())
    .filter(isValidUrl)
    .pop();

console.log(correctUrl);

Shuvo • Nov 4 '21

Nice 💓

MicroTot • Nov 11 '21 • Edited

This is the worst implementation here, but it workd

let url = ["aHR0cHM", "6Ly9kZXYudG", "8vMHNodXZvMC",  "9jb21tZW5",  "0LzFqZTFt" ]
var joinedUrl = url.join()
console.log(joinedUrl)

var result = joinedUrl.replace(/,/g, "")
var decoded = atob(result)
console.log(decoded)

Output:

[LOG]: "https://dev.to/0shuvo0/comment/1je1m"

E. Choroba • Nov 6 '21

A Perl solution I was able to write in less than 5 minutes:

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use MIME::Base64 qw{ decode_base64 };
use List::Util qw{ shuffle };

my $cipher = '[ "6Ly9kZXYudG", "9jb21tZW5", "8vMHNodXZvMC", "aHR0cHM", "0LzFqZTFt" ]';

my @strings = $cipher =~ /"([^"]+)"/g;

my $string = "";
until ($string =~ /^[ -~]+$/) {
    @strings = shuffle(@strings);
    $string = decode_base64(join "", @strings);
}
say $string;

Shuvo • Nov 7 '21

Nice

Comment deleted

Shuvo • Nov 7 '21

Savage

Jeff Lindholm • Nov 10 '21

This sounds like some of the 'oldschool' brain teasers (example: 'Why are manhole covers round?' -- which of course I answer 'because manholes are round, any other shape would be silly')
At least this is asking for you to code something vs. just ask if you know trivia.

Eduard Moraru • Nov 9 '21 • Edited

Here's my not-so-fancy recursive solution :)

let url = [ "6Ly9kZXYudG", "9jb21tZW5", "8vMHNodXZvMC", "aHR0cHM", "0LzFqZTFt" ]

function testUrl(testUrl) {
  return/^[\x00-\x7F]*$/.test(testUrl);
}

function test(segments, currentString) {
  //console.log("try(segments: " + segments + " currentString: " + currentString)
  if (segments.length == 0) {
    let decoded = atob(currentString)
    if (decoded.startsWith("https://") && testUrl(decoded)) {
      console.log("Encoded: " + currentString)
      console.log("Decoded: " + decoded);
    }
  } else {
    for (let i=0; i<segments.length; i++) {
      let newString = currentString + segments[i];
      let remainingSegments = segments.slice(0, i).concat(segments.slice(i+1, segments.length))

      test(remainingSegments, newString)
    }
  }
}

test(url, "")

Outputs:

Encoded: aHR0cHM6Ly9kZXYudG8vMHNodXZvMC9jb21tZW50LzFqZTFt
Decoded: https://dev.to/0shuvo0/comment/1je1m

kb1hyr • Nov 8 '21

Convert the strings in each array element back to base64, then iterate through each possible combination until you find one that, converted back into a string, starts with http. This is aided by the fact the last array element will always end in ==.

Shuvo • Nov 4 '21

You solved it? Great 👏

Shuvo • Nov 4 '21

Have a treat 🍬

GrahamTheDev • Nov 4 '21 • Edited

Sarcastic and snarky comments? 😜🤣

stop being so nice to me, my head won’t fit through the door 😉❤️

d3skdev • Nov 4 '21

My solution:
codepen.io/d3skdev/pen/JjyMQBR?edi...

Shuvo • Nov 4 '21

Seems like your code only works is it elements of array is in a particular order

I shuffled the array again and its not working

let url = `[ "6Ly9kZXYudG", "9jb21tZW5", "8vMHNodXZvMC", "aHR0cHM", "0LzFqZTFt" ]`;

const base64 = JSON.parse(url);
const sort = base64.sort(() => 0 - Math.random()).join("");
const decode = atob(sort);

document.getElementById("url").innerHTML =  decode;

d3skdev • Nov 4 '21

i see, please check my update

Shuvo • Nov 4 '21

It works
Pro tip: you can optimize it by first getting the permutation of the array

jayakrishnancn • Nov 4 '21

Quick question, why did you use sort(() => 0 - Math.random()) ?
to reverse sort?

sarathm09 • Nov 4 '21

No, it shuffles the array elements. Basically he is shuffling the array again and again till he gets it right

Kevin Mack • Nov 8 '21

I fail to see how this interview problem does anything other than play "stump the chump." Realistically interviews are about determining if a person's skills, and personality are a fit for the team, and the work being done. How does this accomplish any of that? I can't think of a single time that this scenario would be applicable to an actual project. And I am of the belief that at any interview, "you are interviewing them, as much as they are interviewing you." This would speak volumes to me as part of a terrible "Know-it-all" / gate keeper culture that I would not want to be a part of.

crimson • Nov 4 '21

I don't want to code but is it...

Turn String back to array (JSON.parse)
Join the array values together
decode the value
Check if it was decoded successfully and if the result was a valid URL
If not, join the array in another sequence and repeat step 2

Can probably shorten it by always putting the string with an equal sign at the end and just looping the rest.

Shuvo • Nov 4 '21

Yes but there might be multiple valid URL tho

Comment deleted

Shuvo • Nov 4 '21

Actually it doesn't decode to dev.to/0shuvo0
And you won't know the expected URL beforehand

Chris Thackrey • Nov 6 '21

This was a fun problem I haven't crossed paths with before, please consider sharing more like this!

Shuvo • Nov 7 '21

Many thanks.

GrahamTheDev • Nov 4 '21

🤣 “great minds think alike”…or is it “fools seldom differ”, I will go for “great minds” as I don’t want to drag you down with me 😜

Mikael Klages • Nov 8 '21

Ignore the input and ddos their site with random urls until you find the correct url.

Personally I would decline to solve their challenge and show them source code I've written in the past, though honestly, this specific question sounds like a big red flag.