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Palantir Technologies
Forward Deployed Software Engineer was asked...November 22, 2017

Merging of Sets: Sets Users which reference other user groups.

6 Answers


Hello! When did you interview?

Beginning of November

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Palantir Technologies

Max sum subsequence of an array

4 Answers

Iterate through that array and comparing the current sum to the maximum sum. If the sum becomes negative at some point, reset it to zero since the maximum sum of the forthcoming elements cannot be larger if you include the negative partial sum. That way, you can solve the problem in O(n) time. It can be speeded up by restarting a new partial sum only on a positive element (or the up to that point largest negative element, which captures the case that all elements are smaller than zero). Less

Most of the answers here are wrong (or perhaps the question is wrong, and a subarray is actually wanted instead). In a subsequence, the array elements do not have to be contiguous, hence simply sum up all the positive numbers in the array. Less

Assuming that array contains positive or negative real numbers. (If numbers are only positive, max sum subsequence is obviously the entire array) O(n log n) divide-and-conquer solution outline: 1. Recursively halve the array into array slices of unit 1. 2. Merge array slices, maintaining the totals and start/end indices of 4 types of subsequences: subsq1. The subsequence that contains the greatest sum within the slice subsq2. The subsequence that contains the greatest sum within the slice and that ends at the rightmost cell of the slice subsq3. The subsequence that contains the greatest sum within the slice and starts at the leftmost cell of the slice subsq4. The entire slice While merging 2 slices together update these slices as follows, 1. max(leftSlice.subsq1, rightSlice.subsq1, leftSlice.subsq2 + rightSlice.subsq3) 2. max(rightSlice.subsq2, leftSlice.subsq2 + rightSlice.subsq4) 3. max(leftSlice.subsq3, rightSlice.subsq3 + leftSlice.subsq4) 4. leftSlice.subsq4 + rightSlice.subsq4 When all the array slices are merged, subsq1 will give you the greatest subsequence. We cannot do better than O(n) because we will have to access each element. We probably cannot do better than O(n log n) because we will have to eventually compare subsequence candidates. Less

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Palantir Technologies

I have 100 bottles of wine. 1 is poisoned. I have 10 rats. My guests are coming in an hour but the poison takes 45 minutes to kick in and "kill" a rat. How do I test for and find the 1 bottle of poisoned wine?

4 Answers

Trivial, for each rat take 10 bottles (rat 1 gets 1-10, 2 gets 11-20....) and feed them a sip from the first bottle, and then the next bottle after a minute, 3rd bottle 2 minutes...etc. If rat n dies after 45+x minutes, bottle 10*n+x is poison Less

label the rats 0 .. 10 label the bottles 0..99 Now, for each bottle: 1) Convert the bottle label to binary number. 2) From the converted number, feed the rats that have their labeled bit set to 1 and leave the rest. (For example: for bottle 2, you will feed rat labeled 1 only) After 45 minutes, check the rats and convert back to decimal. The dead rats are set to 1 and the alive are set to 0. You will get the bottle number. Both this and above solution works for this problem. The above solution works if you have some time freedom and can detect more than one poisoned bottle. This solution can only detect one bottle but works with restricted time. Less

Why open all the wine ! Just open a few bottles and give a bit of each to the first few rats. As long as the rat lives, it's OK. Saves on wine and you'll have more rats for next time. Less

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Palantir Technologies

Rotate the letters of a word by k, using only 1 extra byte of storage.

3 Answers

If I got it right you should be able to do something like this, having k=2: abcde -> deabc In order to do so you have to repeat k time this loop: swap first letter with last one, swap second letter with the last one etc... so if you run this loop the first time you will have eabcd, since you started with abcde -> ebcda -> eacdb -> eabdc -> eabcd, then if you run the loop for the second time (since k=2) you will have eabcd -> dabce -> debca ->deacb -> deabc Less

Using above example abcde: reverse elements after k: abced reverse elements before k: cbaed reverse entire array: deabc Then that one extra byte you're allowed can be used to store the temporary char while swapping elements. Otherwise everything else is done in place. Less

Convert the string to its binary representation. Repeat k times: store the lowest byte in a temporary variable. Shift the string right by 1 byte ( >> 8). Set the highest byte of the string equal to the temporary value. Less

Palantir Technologies

Title Case the given input. Main goal was to check the memory efficiency.

3 Answers

def titleCase(str_): return ' '.join(map(lambda x: chr(ord(x[0]) ^ ord(' '))+x[1:], str_.split(' '))) Less

If strings aren't immutable in your language, the most memory efficient way is to iterate over the string using "indexof(" ",indexOfLastSpace+1)" then replace that exact character by its upper case equivalent. Finding the uppercase equivalent can be done by an helper function like "ToUpper" or is easy to do by yourself if needed (see previous comment with 'A'-'a' which returns by how much you need to increment the char at the position -- also make sure that char is actually a letter by comparing to 'a' and 'z'). Less

public String titleCase(String str){ String[] strar = str.split(" "); StringBuilder strbldr = new StringBuilder(); for (String a : strar) { char [] charArray = a.toCharArray(); charArray[0] = (char)(charArray[0] - ('a' - 'A')); strbldr.append(charArray); strbldr.append(' '); } return strbldr.toString().trim(); } Less

Palantir Technologies

Q: How can you tell if a Linked List is a Palindrome?

3 Answers

First ask if this is a double linked list. If double linked list, simply keep two pointer. One at beginning another at end. Iterate both pointer in each direction and compare at each step. If list is a singlely linkedlist. I would use recursion and pass the head node to the end of the recursion stack call. Compare the head node with the end node at the recursion stack. As the recusion call return, continue advancing the head node with the node in the stack. Less

If it's a singly linked list you can divide the list into two halves, and reverse one, compare if they're the same. This would be in O(n) with O(1) space Less

...poorly. Live and learn!

Palantir Technologies

Given an input string, print out the N most common characters in that string.

2 Answers

import java.util.*; public class freq { public static void frequency(String str, int N) { Map sortedFreqCount = new TreeMap(); for(int i = 0; i entry : sortedFreqCount.entrySet()) { if(printCount == N) break; Character key = entry.getKey(); Integer value = entry.getValue(); System.out.println(printCount + ".) " + key + " -> " + value); printCount++; } } public static > Map sortByValues(final Map map) { Comparator valueComparator = new Comparator() { public int compare(K k1, K k2) { int compare = map.get(k2).compareTo(map.get(k1)); if (compare == 0) return 1; else return compare; } }; Map sortedByValues = new TreeMap(valueComparator); sortedByValues.putAll(map); return sortedByValues; } public static void main(String[] args) { String test = "aaaaaaaaaaaaaaaaaaakkkkkkkkkkkkkkkkkkkddddddddddddhhhhhhhhhbbbbbbbeeeewqqqer"; frequency(test,10); } } Less

Iterate through the string and save each character inside and hashmap, if the character exists already in the map just increase its number of occurrences. once you're done with registering the occurrences of each char, iterate through the map and while iterating create a linked list ordered in decreasing order. Than take the first N elements of the list. The time total time complexity is O(n) with n being the length of the string. This is because all the other operations have a constant time, since you cannot have more than 256 different chars, independently from the lenght of the string. Less

Palantir Technologies

Given a pointer to an element in a singly linked-list, how can you delete the element from the list?

3 Answers

the tricky part in thw questions is that you dont have any pointer to the root of the list. You only have access to the element you want to remove. So you have to copy the data from the next node, and then in doing this you "removed" the element from the list. I hope this is clear Less

Copy the data from the node ahead of the element that has to be deleted and make the current element poi to next->next. Less

Since we have pointer to the element we want to delete and can't go back to it's parent we need to walk the List from the beginning and check current->next == selectedNode once we find a match then all we have to do is current->next = selectedNode->next an example: ROOT > A > B > C > D > E > NULL selectedNode = C while(node != null) { if (node->next == selectedNode) { node->next = selectedNode->next } node = } Less

Palantir Technologies

How would you program a computer to shuffle a deck of cards? (Generate a permutation of the numbers from 1 to 100, uniformly at random)

3 Answers

I think you can use some group theory to solve this as well. Pick random number a in [0,51]. Pick another number b in [1,51] such that the gcd(52,b) = 1. The nth card in the permutation is the card at the index given by a+bn % 52, where % means modulus. i.e. nth index = a+bn (mod 52) In addition, this method works for any permutation of any length (assuming random distribution of b values). Less

Easy way : assign a random number to each card (O(n), possibly by using an intermediate object of some kind like a Tuple), sort by this random number (O(n.log(n)) and you're done (final complexity O(n.log(n))). I don't see a more efficient way to do it without breaking the random distribution, but a deck of cards rarely have more than k*52 cards (with k small), so it wouldn't really matter anyway in the real world. Less

Make array of numbers from 1 to 52. Go through each of cards each time removing one number randomly chosen from array and assigning it to the card. Sort the cards by numbers assigned to cards Less


Please write a generic function that performs a specific operation on time series data.

3 Answers

On a white board in code format.

Hi Please elaborate the question more like what language and what were they looking for exactly. Thanks Less

Will you kindly suggest some resources for getting past the Technical screening part? I failed the Tech. Screening. But, I have a chance to attempt the interview one other time next month. Any inputs will be extremely helpful for getting past the Tech Screen this time around. Thank you. Less

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