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Interview Qs Decoded - # 2

Cat on June 11, 2020

I wanted to share with you all an actual challenge I was given during an interview! The interviewer said, "Alright, let's do some string reversals...

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Arty ‽ • Jun 12 '20 • Edited

Nice use of pop()!
I would not have think of that...
Here's how I went about it :

let str = 'please reverse me';
let newStr = '' ;
for(let i= str.length -1;i >=0;i--){
newStr += str[i]
}
console.log(newStr)

JP Antunes • Jun 12 '20

Using recursion so that only half the string is looped over:

const strReverso = ([...str], i = 0) => {
    return i < str.length / 2
            ? ([ str[i], str[str.length - 1 - i] ] = [ str[str.length - 1 - i], str[i] ], strReverso(str, ++i))
            : str.join('');
}

Het Shah • Jun 12 '20 • Edited

How about this solution?

const reverse = ([...str]) => {
  const len = str.length;
  for(let i=0;i<len/2;i++){
    [str[i],str[len-i]] = [str[len-i],str[i]];
  }

  return str.join('');
}

Het Shah • Jun 12 '20

Or not so optimized but shorter code:

const reverse2 = ([...str]) => 
    str.map((ch,i)=>str[str.length-1-i]).join('');

Matt Hamilton • Jun 12 '20

In Python:

More verbose using a for loop:

def revFunction(str):
  arr = []
  for i in range(len(str)):
    arr.append(str[-i-1])
  return ''.join(arr)

More concise, using a list comprehension:

def revFunction(str):
  return ''.join([str[-i-1] for i in range(len(str))])

Cat • Jun 13 '20

Yesss I was waiting for a Python response!!!

Matt Hamilton • Jun 13 '20

Actually just realised an even more concise way in Python. The slice operator takes a step as an optional 3rd argument, and I just found out the step can be negative.

>>> str = "Reverse me!"
>>> str[::-1]
'!em esreveR'

So...

def revFunction(str):
  return str[::-1]

Yavuz Tas • Jun 13 '20

Here is my solution a little bit different. Sacrifices memory in favor of speed, reversing a string in log(n) time complexity :)

function reverseByDivide(input){
  if(input.length < 2)
    return input
  var left = input.substring(0, input.length/2)
  var right = input.substring(input.length/2, input.length)
  return reverseByDivide(right) + reverseByDivide(left)
}

Gracie Gregory (she/her) • Jun 12 '20

I love this series!

Ryan Dsouza • Jun 12 '20 • Edited

Great one! I though have a slightly different take if you don't want to create an intermediate array :)

function revFunction(str){
   const reversedString = [];
   for(let i = str.length - 1; i >= 0; i--){
       reversedString.push(str[i]);
   }
   return reversedString.join('');
};

Mike Talbot ⭐ • Jun 12 '20 • Edited

It's an interesting problem because it drives for a more optimal solution rather than desiring some super terse response I think. Though clearly .reverse() is highly optimised and terse! On DEV we often see solutions to problems written deliberately functionally (I do this a lot). Asking for no .reduce() definitely pushes us to realise we are being asked for an actual function not some kind of super expression and we should therefore be able to strive for optimal performance and memory use.

I run a lot of interviews, in this question I'd be looking for the following in the answer:

Don't use string appending - if the string was long this is a very slow solution - the result should concatenate an output array using .join()
Don't build an array using the spread operator at each step, it's even worse than string appending for performance and memory usage/garbage collection.
An optimal solution will cache commonly dereferenced properties to improve performance

Along with another commenter I also like your use of pop() very smart that. I think there's a slight error in it though - surely your function adds one too many characters? <=str.length - overall your solution is great.

Minimised space usage would have me making this hybrid of solutions mentioned:

function reverseString(str) {
     const source = str.split('')
     const swap = source.length >> 1
     for(let i = 0, j = source.length-1; i < swap;i++,j--) {
         let a = source[i]
         source[i] = source[j]
         source[j] = a
     }
     return source.join('')
}

Writing out that variable swap rather than using [source[i], source[j]] = [source[j], source[i]] avoids creating 2 arrays on every loop. These would be quickly garbage collected but it depends on the browser as to just how much memory/time that uses.
This has only one array being used and so reduces allocations

Cat • Jun 13 '20

I actually used splitString.length at first during the interview, but I wasn't getting the right result-- I came up short. Then I realized I needed to use str.length instead to get the desired result, and it was what the interviewer was looking for.

Thank you for your well-thought-out, detailed comment! I will keep those pointers in mind during the next technical interview.

Mike Talbot ⭐ • Jun 13 '20

Ah well I'd have made that mistake as well then :) Fair enough!

Johannes Millan • Jun 13 '20 • Edited

Performance is one way to look at it, another is readability and ease of use. If you assume only reasonably short strings and a reasonable amount of iterations then I would argue that the easier readable solution is the better one, as it is inviting less misunderstandings and saves time every time a developer reads your code. Just mentioning that so you don't judge your interviewees prematurely only on performance ;)

mikeyGlitz • Jun 12 '20 • Edited

For those who favor a more functional approach:

const str = 'Please reverse me';
const reverse = str.split('').reduce((acc, curr) => [curr, ...acc], []).join();
console.info(reverse);

Output:

em esreveR esaelP

Cat • Jun 12 '20

Thanks for sharing, but one of the stipulations was that you can’t use reduce! 😅

mikeyGlitz • Jun 12 '20

My bad -- missed that part.

Cat • Jun 12 '20

That’s okay! Your solution is pretty slick. I was thinking of using reduce until he said not to haha.

Carlos A. • Jun 14 '20

XD
Is so cool to read so many possibilities and some are quite elegant. Though it is kind of a simple example, as you have mentioned, the goal is to show your communication skills and your language knowledge.

const testText = "Please, reverse me!";
const reverseFunction = text => {
  const arrayText = Array.from(text)
  const reversedText = [];
  arrayText.forEach(elem => {
    reversedText.unshift(elem)
  });
  return reversedText.join('');
}

Here's what came to my mind. Though, I wonder about the JS version, could that be a valid question? Don't forget to review and clean your code. Finally, don't stay quiet while developing, speak your thought process, so your interviewer can follow you.

Again, thank you for the chance. =)

Harry Balls • Jun 12 '20

Pretty cool. Check my solution.

private static object ReverseString(string input)
{
var str = "";
int index = input.Length - 1;
while (index >= 0)
{
str += $"{input[index--]}";
}
return str;
}

Comment deleted

Cat • Jun 12 '20 • Edited

I thought a string was a string by default. If you unshift() a string, would that throw an error?

I’m not by a computer right now to try it out lol