Interview Q&A

LinkedList<T> is a doubly linked list collection in C#. It stores elements as nodes,

where each node contains the data and references to the previous and next nodes.

• Allows efficient insertions and deletions anywhere in the list.
• Does not support indexed access like List<T>.

Example:

LinkedList<int> numbers = new LinkedList<int>();

numbers.AddLast(10);

numbers.AddLast(20);

To implement a custom collection:

• Derive from existing base classes like Collection<T>, List<T>, or implement

interfaces such as ICollection<T>, IEnumerable<T>, or IList<T>.

• Override or implement necessary methods like Add(), Remove(),

GetEnumerator(), and indexers.

• Provide custom behavior, validation, or constraints as needed.

Example:

public class MyCustomCollection<T> : Collection<T>
{

protected override void InsertItem(int index, T item)

{

// Custom validation

if (item == null) throw new

rgumentNullException(nameof(item));

base.InsertItem(index, item);

}
}

• Use the Stopwatch class from System.Diagnostics to time operations

ccurately.

• Profile your code using tools like Visual Studio Profiler, dotTrace, or PerfView for

deeper insights.

• Measure specific operations like add, remove, search, or iteration by running them

multiple times and averaging results.

Example:

var stopwatch = Stopwatch.StartNew();
list.Add(1000);

stopwatch.Stop();

Console.WriteLine($"Add operation took {stopwatch.ElapsedTicks}

ticks");

ConcurrentDictionary<TKey, TValue> is a thread-safe dictionary designed for

concurrent access by multiple threads without needing external synchronization (locks).

• Supports atomic operations like adding, updating, and removing items.
• Useful in multi-threaded scenarios where data integrity and performance are critical.

Example:

ConcurrentDictionary<int, string> concurrentDict = new

ConcurrentDictionary<int, string>();

concurrentDict.TryAdd(1, "One");

concurrentDict.TryUpdate(1, "Uno", "One");

Collection initializers allow you to create and populate a collection in a concise way at the

time of declaration.

Example:

List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };
Dictionary<string, int> ages = new Dictionary<string, int>
{

{ "Alice", 30 },

{ "Bob", 25 }

};

This syntax internally calls the collection’s Add() method for each element.

SortedSet<T> is a collection that stores unique elements in sorted order.

• Implements a self-balancing binary search tree (usually a Red-Black Tree).
• Automatically maintains elements in ascending sorted order.
• Provides set operations like union, intersection, and difference.

Example:

SortedSet<int> sortedSet = new SortedSet<int> { 5, 1, 3 };

sortedSet.Add(2); // Sorted order maintained: {1, 2, 3, 5}

SortedList<TKey, TValue> is a collection of key-value pairs that maintains the

elements sorted by keys.

• Implements both IDictionary<TKey, TValue> and

IList<KeyValuePair<TKey, TValue>>.

• Keys are automatically sorted in ascending order based on their natural comparer

or a provided comparer.

Example:

SortedList<int, string> sortedList = new SortedList<int, string>();

sortedList.Add(3, "Three");

sortedList.Add(1, "One");

sortedList.Add(2, "Two");

The elements are stored sorted by key: 1, 2, 3.

LinkedList<T> is a doubly linked list collection in C#. It stores elements as nodes,

where each node contains the data and references to the previous and next nodes.

• Allows efficient insertions and deletions anywhere in the list.
• Does not support indexed access like List<T>.

Example:

LinkedList<int> numbers = new LinkedList<int>();

numbers.AddLast(10);

numbers.AddLast(20);

int MatrixChainOrder(int[] dims)
{
int n = dims.Length - 1;
int[,] dp = new int[n, n];
for (int l = 2; l <= n; l++)
{

Follow on:

for (int i = 0; i < n - l + 1; i++)
{
int j = i + l - 1;
dp[i, j] = int.MaxValue;
for (int k = i; k < j; k++)
{
int cost = dp[i, k] + dp[k + 1, j] + dims[i] *

dims[k + 1] * dims[j + 1];

dp[i, j] = Math.Min(dp[i, j], cost);
}
}
}
return dp[0, n - 1];
}

Explanation:

DP calculates minimal cost to multiply chain of matrices by trying all partitions.

rray.Reverse(words); string result = string.Join(" ", words);

What interviewers expect

• A clear definition tied to Coding Scenarios in C# Coding Interview projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Coding Interview application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# Coding Interview architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: rray.Reverse(matrix[i]); } } Explanation: Transpose matrix and then reverse each row to rotate 90° clockwise.

What interviewers expect

• A clear definition tied to Coding in C# Coding Interview projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Coding Interview application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# Coding Interview architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

public void RotateMatrix(int[][] matrix) {

int n = matrix.Length;

// Transpose

for (int i = 0; i < n; i++) {

for (int j = i + 1; j < n; j++) {

int temp = matrix[i][j];

matrix[i][j] = matrix[j][i];

matrix[j][i] = temp;

// Reverse each row

for (int i = 0; i < n; i++) {

Array.Reverse(matrix[i]);

Explanation:

Transpose matrix and then reverse each row to rotate 90° clockwise.

Follow on:

List<string> GenerateParenthesis(int n)
{
List<string> result = new List<string>();

Generate("", 0, 0, n, result);

return result;
}

void Generate(string current, int open, int close, int max,

List<string> result)
{
if (current.Length == max * 2)
{

result.Add(current);

return;
}
if (open < max)

Generate(current + "(", open + 1, close, max, result);

if (close < open)

Generate(current + ")", open, close + 1, max, result);

}

Explanation:

Use backtracking to add '(' and ')' only when valid.

long LargestPrimeFactor(long n)

{
long maxPrime = -1;

while (n % 2 == 0)

{
maxPrime = 2;
n /= 2;
}
for (long i = 3; i * i <= n; i += 2)
{

while (n % i == 0)

{
maxPrime = i;
n /= i;
}
}
if (n > 2) maxPrime = n;
return maxPrime;
}

Follow on:

Explanation:

Divide out factors of 2, then test odd factors; leftover > 2 is prime.

Miscellaneous Problems

Integers
int FindMissingNumber(int[] nums)
{
int low = 0, high = nums.Length - 1;

while (low <= high)

{
int mid = low + (high - low) / 2;
if (nums[mid] == mid)
low = mid + 1;

else

high = mid - 1;
}
return low;
}

Explanation:

In perfect array nums[i] == i; missing number breaks this property, use binary search to find

breakpoint.

Mathematical Problems

bool CanPartition(int[] nums)

{
int sum = nums.Sum();

Follow on:

if (sum % 2 != 0) return false;
int target = sum / 2;
bool[] dp = new bool[target + 1];
dp[0] = true;
foreach (int num in nums)
{
for (int j = target; j >= num; j--)
{
dp[j] = dp[j] || dp[j - num];
}
}
return dp[target];
}

Explanation:

Subset sum to check if half the total sum is achievable.

Sorting and Searching

Answer: bool IsPalindrome(string str) { int left = 0, right = str.Length - 1; while (left &lt; right) { if (str[left] != str[right]) return false; left++; right--; } return true; }

What interviewers expect

• A clear definition tied to Coding Scenarios in C# Coding Interview projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Coding Interview application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# Coding Interview architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: int[] arr = { 3, 5, 1, 5, 2 }; HashSet&lt;int&gt; set = new HashSet&lt;int&gt;(); foreach (int num in arr) { if (!set.Add(num)) { Console.WriteLine(num); break; } }

What interviewers expect

• A clear definition tied to Coding Scenarios in C# Coding Interview projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Coding Interview application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# Coding Interview architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

int[] a = { 1, 3, 5 };
int[] b = { 2, 4, 6 };
int i = 0, j = 0;
List<int> result = new List<int>();

while (i < a.Length && j < b.Length)

result.Add(a[i] < b[j] ? a[i++] : b[j++]);

while (i < a.Length) result.Add(a[i++]);

while (j < b.Length) result.Add(b[j++]);

Answer: string sentence = "CSharp makes backend development powerful"; string[] words = sentence.Split(' '); string longest = words[0]; foreach (string word in words) { if (word.Length &gt; longest.Length) longest = word; }

What interviewers expect

• A clear definition tied to Coding Scenarios in C# Coding Interview projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Coding Interview application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# Coding Interview architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

List<object> list = new List<object> { 1, new List<int> { 2, 3 }, 4

};

List<int> result = new List<int>();

void Flatten(List<object> input)

{
foreach (var item in input)
{
if (item is int)

result.Add((int)item);

else

Flatten((List<object>)item);

}
}

rr[j] = arr[j + 1]; rr[j + 1] = temp; } } }

What interviewers expect

• A clear definition tied to Coding Scenarios in C# Coding Interview projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Coding Interview application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# Coding Interview architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: string input = "DotNet"; Dictionary&lt;char, int&gt; map = new Dictionary&lt;char, int&gt;(); foreach (char c in input.ToLower()) map[c] = map.ContainsKey(c) ? map[c] + 1 : 1;

What interviewers expect

• A clear definition tied to Coding Scenarios in C# Coding Interview projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Coding Interview application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# Coding Interview architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: bool IsSorted(int[] arr) { for (int i = 0; i &lt; arr.Length - 1; i++) { if (arr[i] &gt; arr[i + 1]) return false; } return true; }

What interviewers expect

• A clear definition tied to Coding Scenarios in C# Coding Interview projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Coding Interview application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# Coding Interview architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

string input = "Dot@Net#2024!";
StringBuilder sb = new StringBuilder();
foreach (char c in input)
{
if (char.IsLetterOrDigit(c))

sb.Append(c);

}
string result = sb.ToString();

Final Notes

• These questions frequently appear in L1/L2 interviews
• They test logic clarity, memory, and problem-solving
• Ideal for LinkedIn posts, reels, ebooks, and interviews