Demystifying TestDom

Recruiters nowadays use online timed tests when screening developers. I recently looked at Python & Algorithms Hard questions at TestDome. While the timing and hints seem to push towards implementing tricks from scratch, for the quality in long term it is better to structure the problem and use established solutions (divide & conquer). The battery of “hard” problems can be solved in few lines of code, using the standard Python libraries (Numpy, Pandas, Collections).

League Table (Hard)
The task is to essentially sort the football results by multiple criteria: first by the higher number of scores, next (breaking ties) by the lower number of games, finally (breaking ties) by the default order. While the hint suggests building a custom comparator, the effective way is by Pandas tables (5 lines).

import pandas as pd

def player_rank(self, rank):
        ts = []
        ts = map(lambda t:(t[1][0],t[1][1]['score'],t[1][1]['games_played'],t[0]),enumerate(self.standings.items()))
        df = pd.DataFrame(ts)
        df = df.sort_values(by=[1,2,3],ascending=[False,True,True]).reset_index()
        return df.loc[rank-1][0]

Sorted Search (Hard)
The task is to determine how many values in a sorted list are less than the given value. This is equivalent to finding the right position to insert (to maintain the sorted order). While the solution suggests playing with a variant of binary search on your own, the effective way is by the Python bisect algorithm (1 line).

from bisect import bisect_left

def count_numbers(sorted_list, less_than):
    return bisect_left(sorted_list,less_than)

Train Composition (Hard)
The task is to implement a data structure that mimics composing the train: one can quickly attach and detach to/from both ends. The effective way is to use Python double-ended queue (5 lines).

from collections import deque

class TrainComposition:
    
    def __init__(self):
        self.d = deque()
    
    def attach_wagon_from_left(self, wagonId):
        self.d.appendleft(wagonId)
    
    def attach_wagon_from_right(self, wagonId):
        self.d.append(wagonId)

    def detach_wagon_from_left(self):
        return self.d.popleft()
    
    def detach_wagon_from_right(self):
        return self.d.pop()