This C# Algorithms and SQL Online Test
Separates Good From Bad Hires

Binary search tree (BST) is a binary tree where the value of each node is larger or equal to the values in all the nodes in that node's left subtree and is smaller than the values in all the nodes in that node's right subtree.

Write a function that, efficiently with respect to time used, checks if a given binary search tree contains a given value.

For example, for the following tree:

• n1 (Value: 1, Left: null, Right: null)
• n2 (Value: 2, Left: n1, Right: n3)
• n3 (Value: 3, Left: null, Right: null)

Call to Contains(n2, 3) should return true since a tree with root at n2 contains number 3.

Given the following data definition, write a query that returns the number of students whose first name is John.

TABLE students
   id INTEGER PRIMARY KEY,
   firstName VARCHAR(30) NOT NULL,
   lastName VARCHAR(30) NOT NULL

A company is designing the class hierarchy for various cache implementations:

public class Cache {}

public class DiskCache : Cache {}

public class MemoryCache : Cache {}

public class OptimizedDiskCache : DiskCache {}

Select all the answers that will result in a runtime exception.

A table containing the students enrolled in a yearly course has incorrect data in records with ids between 20 and 100 (inclusive).

TABLE enrollments
  id INTEGER NOT NULL PRIMARY KEY
  year INTEGER NOT NULL
  studentId INTEGER NOT NULL

Write a query that updates the field 'year' of every faulty record to 2015.

Information about pets is kept in two separate tables:

TABLE dogs
  id INTEGER NOT NULL PRIMARY KEY,
  name VARCHAR(50) NOT NULL

TABLE cats
  id INTEGER NOT NULL PRIMARY KEY,
  name VARCHAR(50) NOT NULL

Write a query that select all distinct pet names.

See the example case for more details.

A new social network site has the following data tables:

users
id	name	sex
1	Ann	null
2	Steve	m
3	Mary	f
4	Brenda	f

friends
user1	user2
1	2
1	3
2	3

Select data that will be returned by the following SQL query:

SELECT users.name, COUNT(*) as count FROM users
LEFT JOIN friends
ON users.id = friends.user1 OR users.id = friends.user2
WHERE users.sex = 'f'
GROUP BY users.id, users.name;

App usage data are kept in the following table:

TABLE sessions
  id INTEGER PRIMARY KEY,
  userId INTEGER NOT NULL,
  duration DECIMAL NOT NULL

Write a query that selects userId and average session duration for each user who has more than one session.

See the example case for more details.

Each item in a web shop belongs to a seller. To ensure service quality, each seller has a rating.

The data are kept in the following two tables:

TABLE sellers
  id INTEGER PRIMARY KEY,
  name VARCHAR(30) NOT NULL,
  rating INTEGER NOT NULL

TABLE items
  id INTEGER PRIMARY KEY,
  name VARCHAR(30) NOT NULL,
  sellerId INTEGER
  FOREIGN KEY (sellerId) REFERENCES sellers(id)

Write a query that selects the item name and the name of its seller for each item that belongs to a seller with a rating greater than 4. The query should return the name of the item as the first column and name of the seller as the second column.

See the example case for more details.

Refactor the AlertService and AlertDAO classes:

• Create a new interface, named IAlertDAO, that contains the same methods as AlertDAO.
• AlertDAO should implement the IAlertDAO interface.
• AlertService should have a public constructor that accepts IAlertDAO.
• The RaiseAlert and GetAlertTime methods should use the object passed through the constructor.

Implement the UniqueNames method. When passed two arrays of names, it will return an array containing the names that appear in either or both arrays. The returned array should have no duplicates.

For example, calling MergeNames.UniqueNames(new string[]{'Ava', 'Emma', 'Olivia'}, new string[]{'Olivia', 'Sophia', 'Emma'}) should return an array containing Ava, Emma, Olivia, and Sophia in any order.

Implement the FindRoots function to find the roots of the quadratic equation: ax² + bx + c = 0. If the equation has only one solution, the function should return that solution as both elements of the tuple. The equation will always have at least one solution.

The roots of the quadratic equation can be found with the following formula:

For example, the roots of the equation 2x² + 10x + 8 = 0 are -1 and -4.

A playlist is considered a repeating playlist if any of the songs contain a reference to a previous song in the playlist. Otherwise, the playlist will end with the last song which points to null.

Implement a function IsRepeatingPlaylist that, efficiently with respect to time used, returns true if a playlist is repeating or false if it is not.

For example, the following code prints "True" as both songs point to each other.

Song first = new Song("Hello");
Song second = new Song("Eye of the tiger");
    
first.NextSong = second;
second.NextSong = first;

Console.WriteLine(first.IsRepeatingPlaylist());

User interface contains two types of user input controls: TextInput, which accepts all characters and NumericInput, which accepts only digits.

Implement the class TextInput that contains:

• Public method void Add(char c) - adds the given character to the current value
• Public method string GetValue() - returns the current value

Implement the class NumericInput that:

• Inherits TextInput
• Overrides the Add method so that each non-numeric character is ignored

For example, the following code should output "10":

TextInput input = new NumericInput();
input.Add('1');
input.Add('a');
input.Add('0');
Console.WriteLine(input.GetValue());

Using only NUnitLite 1's Assert.AreEqual method, write tests for the Account class that cover the following cases:

• The Deposit and Withdraw methods will not accept negative numbers.
• Account cannot overstep its overdraft limit.
• The Deposit and Withdraw methods will deposit or withdraw the correct amount, respectively.
• The Withdraw and Deposit methods return the correct results.

public class Account
{
    public double Balance { get; private set; }
    public double OverdraftLimit { get; private set; }

    public Account(double overdraftLimit)
    {
        this.OverdraftLimit = overdraftLimit > 0 ? overdraftLimit : 0;
        this.Balance = 0;
    }

    public bool Deposit(double amount)
    {
        if (amount >= 0)
        {
            this.Balance += amount;
            return true;
        }
        return false;
    }

    public bool Withdraw(double amount)
    {
        if (this.Balance - amount >= -this.OverdraftLimit && amount >= 0)
        {
            this.Balance -= amount;
            return true;
        }
        return false;
    }
}

Each of the test case methods should be in the Tester class and have the Test attribute.

Write a function that, when passed a list and a target sum, returns, efficiently with respect to time used, two distinct zero-based indices of any two of the numbers, whose sum is equal to the target sum. If there are no two numbers, the function should return null.

For example, FindTwoSum(new List<int>() { 3, 1, 5, 7, 5, 9 }, 10) should return a Tuple<int, int> containing any of the following pairs of indices:

• 0 and 3 (or 3 and 0) as 3 + 7 = 10
• 1 and 5 (or 5 and 1) as 1 + 9 = 10
• 2 and 4 (or 4 and 2) as 5 + 5 = 10

Each account on a website has a set of access flags that represent a users access.

Update and extend the enum so that it contains three new access flags:

1. A Writer access flag that is made up of the Submit and Modify flags.
2. An Editor access flag that is made up of the Delete, Publish and Comment flags.
3. An Owner access that is made up of the Writer and Editor flags.

For example, the code below should print "False" as the Writer flag does not contain the Delete flag.

Console.WriteLine(Access.Writer.HasFlag(Access.Delete))

The following data definition defines an organization's employee hierarchy.

An employee is a manager if any other employee has their managerId set to this employee's id. That means John is a manager if at least one other employee has their managerId set to John's id.

TABLE employees
  id INTEGER NOT NULL PRIMARY KEY
  managerId INTEGER
  name VARCHAR(30) NOT NULL
  FOREIGN KEY (managerId) REFERENCES employees(id)

Write a query that selects only the names of employees who are not managers.

See the example case for more details.

The following two tables are used to define users and their respective roles:

TABLE users
  id INTEGER NOT NULL PRIMARY KEY,
  userName VARCHAR(50) NOT NULL

TABLE roles
  id INTEGER NOT NULL PRIMARY KEY,
  role VARCHAR(20) NOT NULL

The users_roles table should contain the mapping between each user and their roles. Each user can have many roles, and each role can have many users.

Modify the provided SQL create table statement so that:

• Only users from the users table can exist within users_roles.
• Only roles from the roles table can exist within users_roles.
• A user can only have a specific role once.

See the example case for more details.

Implement methods and properties in the DecoratorStream class:

• Write method should write the prefix into the underlying stream member only on the first Write invocation. It should always write the bytes it receives to the underlying stream.
• The prefix should be written in UTF-8 encoding.

For example, if the DecoratorStream is instantiated with "First line: " as the prefix parameter and Write method is called with UTF-8 byte representation of "Hello, world!", it should write "First line: Hello, world!" into the underlying stream.

Implement a function FolderNames, which accepts a string containing an XML file that specifies folder structure and returns all folder names that start with startingLetter. The XML format is given in the example below.

For example, for the letter 'u' and XML file:

<?xml version="1.0" encoding="UTF-8"?>
<folder name="c">
    <folder name="program files">
        <folder name="uninstall information" />
    </folder>
    <folder name="users" />
</folder>

the function should return "uninstall information" and "users" (in any order).

Implement function CountNumbers that accepts a sorted array of unique integers and, efficiently with respect to time used, counts the number of array elements that are less than the parameter lessThan.

For example, SortedSearch.CountNumbers(new int[] { 1, 3, 5, 7 }, 4) should return 2 because there are two array elements less than 4.

A TrainComposition is built by attaching and detaching wagons from the left and the right sides, efficiently with respect to time used.

For example, if we start by attaching wagon 7 from the left followed by attaching wagon 13, again from the left, we get a composition of two wagons (13 and 7 from left to right). Now the first wagon that can be detached from the right is 7 and the first that can be detached from the left is 13.

Implement a TrainComposition that models this problem.

An insurance company maintains records of sales made by its employees. Each employee is assigned to a state. States are grouped under regions. The following tables contain the data:

TABLE regions
  id INTEGER PRIMARY KEY
  name VARCHAR(50) NOT NULL

TABLE states
  id INTEGER PRIMARY KEY
  name VARCHAR(50) NOT NULL
  regionId INTEGER NOT NULL
  FOREIGN KEY (regionId) REFERENCES regions(id)

TABLE employees
  id INTEGER PRIMARY KEY
  name VARCHAR(50) NOT NULL
  stateId INTEGER NOT NULL
  FOREIGN KEY (stateId) REFERENCES states(id)

TABLE sales
  id INTEGER PRIMARY KEY
  amount INTEGER NOT NULL
  employeeId INTEGER NOT NULL
  FOREIGN KEY (employeeId) REFERENCES employees(id)  

Management requires a comparative region sales analysis report.

Write a query that returns:

• The region name.
• Average sales per employee for the region (Average sales = Total sales made for the region / Number of employees in the region).
• The difference between the average sales of the region with the highest average sales, and the average sales per employee for the region (average sales to be calculated as explained above).

Employees can have multiple sales. A region with no sales should be also returned. Use 0 for average sales per employee for such a region when calculating the 2nd and the 3rd column.

See the example case for more details.

As a part of the route planner, the RouteExists method is used as a quick filter if the destination is reachable, before using more computationally intensive procedures for finding the optimal route.

The roads on the map are rasterized and produce a matrix of boolean values - true if the road is present or false if it is not. The roads in the matrix are connected only if the road is immediately left, right, below or above it.

Finish the RouteExists method so that it returns true if the destination is reachable or false if it is not. The fromRow and fromColumn parameters are the starting row and column in the mapMatrix. The toRow and toColumn are the destination row and column in the mapMatrix. The mapMatrix parameter is the above mentioned matrix produced from the map.

For example, for the given rasterized map, the code below should return true since the destination is reachable:

bool[,] mapMatrix = {
	{true, false, false},
	{true, true, false},
	{false, true, true}
};

RouteExists(0, 0, 2, 2, mapMatrix);