Interview Q&A

public int FindDuplicate(int[] nums) {
int slow = nums[0], fast = nums[0];

do {

slow = nums[slow];
fast = nums[nums[fast]];
} while (slow != fast);
fast = nums[0];

while (slow != fast) {

slow = nums[slow];
fast = nums[fast];
}
return slow;
}

Explanation:

Floyd’s Tortoise and Hare cycle detection, treating values as pointers.

int MinCutPalindromePartition(string s) {
int n = s.Length;
bool[,] dp = new bool[n, n];
int[] cuts = new int[n];
for (int i = 0; i < n; i++) {
int minCuts = i;
for (int j = 0; j <= i; j++) {
if (s[j] == s[i] && (i - j < 2 || dp[j + 1, i - 1])) {
dp[j, i] = true;

minCuts = j == 0 ? 0 : Math.Min(minCuts, cuts[j - 1]

+ 1);

}
}
cuts[i] = minCuts;
}
return cuts[n - 1];
}

bool PrintAncestors(TreeNode root, int target) {

if (root == null) return false;
if (root.val == target) return true;
if (PrintAncestors(root.left, target) ||

PrintAncestors(root.right, target)) {

Console.Write(root.val + " ");

return true;
}
return false;
}

Explanation:

Recursive check if target is in subtree; print current node on path back if yes.

Answer: 1 : -1; return candidate; Explanation: Boyer-Moore Voting Algorithm tracks majority element by counting net votes. Follow on:

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.

Answer: x * temp * temp : (temp * temp) / x; Explanation: Recursive fast power divides exponent by 2 to reduce complexity to O(log n).

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.

Answer: 1 : -1; return candidate; Explanation: Maintain a candidate and count; majority element survives this cancellation.

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.

int MaxSubArray(int[] nums)
{
int maxSoFar = nums[0];
int maxEndingHere = nums[0];
for (int i = 1; i < nums.Length; i++)
{
maxEndingHere = Math.Max(nums[i], maxEndingHere + nums[i]);
maxSoFar = Math.Max(maxSoFar, maxEndingHere);
}
return maxSoFar;
}

Explanation:

Track max subarray ending at current position; update global max.

public sealed class Singleton
{
private static readonly object lockObj = new object();
private static Singleton instance;
private Singleton() { }
public static Singleton Instance
{

get

{
if (instance == null)
{

lock (lockObj)

{
if (instance == null)
instance = new Singleton();
}
}
return instance;
}
}
}

Answer: rray.Copy(nums, dp, n); for (int i = 1; i < n; i++) { for (int j = 0; j < i; j++) { Follow on: if (nums[i] > nums[j]) dp[i] = Math.Max(dp[i], dp[j] + nums[i]); } } return dp.Max(); }

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.

bool IsValidBST(TreeNode root) {

return Validate(root, null, null);
}

Follow on:

bool Validate(TreeNode node, int? min, int? max) {

if (node == null) return true;
if ((min != null && node.val <= min) || (max != null && node.val
>= max)) return false;
return Validate(node.left, min, node.val) &&

Validate(node.right, node.val, max);

}

Explanation:

Pass down min and max bounds for subtree values; node must be in (min, max) range.

public int LengthOfLongestSubstringTwoDistinct(string s) {
int left = 0, right = 0, maxLen = 0;
Dictionary<char, int> map = new Dictionary<char, int>();

while (right < s.Length) {

Follow on:

char c = s[right];
map[c] = right;
if (map.Count > 2) {
int delIndex = map.Values.Min();

map.Remove(s[delIndex]);

left = delIndex + 1;
}
maxLen = Math.Max(maxLen, right - left + 1);

right++;

}
return maxLen;
}

Explanation:

Sliding window with hashmap to track indices of distinct chars, remove the leftmost when

>2.

int MaxSumIncreasingSubsequence(int[] nums) {

int n = nums.Length;

int[] dp = new int[n];

Array.Copy(nums, dp, n);

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

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

Follow on:

if (nums[i] > nums[j])

dp[i] = Math.Max(dp[i], dp[j] + nums[i]);

return dp.Max();

min, int? max) {

if (node == null) return true;

if ((min != null && node.val <= min) || (max != null && node.val

>= max)) return false;

return Validate(node.left, min, node.val) &&

Validate(node.right, node.val, max);

Explanation:

Pass down min and max bounds for subtree values; node must be in (min, max) range.

void RotateMatrix(int[][] matrix)

{
int n = matrix.Length;

// Transpose

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

(matrix[i][j], matrix[j][i]) = (matrix[j][i],

matrix[i][j]);

// Reverse each row

for (int i = 0; i < n; i++)
{
int left = 0, right = n - 1;

while (left < right)

{

(matrix[i][left], matrix[i][right]) = (matrix[i][right],

matrix[i][left]);

left++;

right--;

}
}
}

Explanation:

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

Answer: int CountOnes(int n) { int count = 0; while (n != 0) { n &amp;= (n - 1); // drops the lowest set bit count++; } return count; } Explanation: Brian Kernighan’s algorithm removes one set bit per iteration.

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.

int KthSmallest(int[][] matrix, int k)
{
int n = matrix.Length;
int low = matrix[0][0], high = matrix[n - 1][n - 1];

while (low < high)

{

Follow on:

int mid = low + (high - low) / 2;
int count = 0, j = n - 1;
for (int i = 0; i < n; i++)
{

while (j >= 0 && matrix[i][j] > mid)

j--;

count += (j + 1);
}
if (count < k)
low = mid + 1;

else

high = mid;
}
return low;
}

Explanation:

Binary search on values, count how many elements ≤ mid using matrix’s sorted rows/cols.

int RodCutting(int[] prices, int n)
{
int[] dp = new int[n + 1];
dp[0] = 0;

Follow on:

for (int i = 1; i <= n; i++)
{
int maxVal = int.MinValue;
for (int j = 0; j < i; j++)
{
maxVal = Math.Max(maxVal, prices[j] + dp[i - j - 1]);
}
dp[i] = maxVal;
}
return dp[n];
}

Explanation:

Max revenue by cutting rod into pieces of various lengths.

ctualSum += num; int missing = expectedSum - actualSum;

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.

double Calculate(double a, double b, char op)
{
return op switch
{

'+' => a + b,

Follow on:

'-' => a - b,

'*' => a * b,

'/' => b != 0 ? a / b : throw new DivideByZeroException(),

_ => throw new ArgumentException("Invalid operator"),

};

}

Explanation:

Simple switch statement performing basic arithmetic, with divide-by-zero check.

public bool IsBalanced(string s) {
Stack<char> stack = new Stack<char>();
Dictionary<char, char> pairs = new Dictionary<char, char> {

{')', '('}, {']', '['}, {'}', '{'}

};

foreach (char c in s) {
if ("([{".Contains(c))

stack.Push(c);

else if (")]}".Contains(c)) {

if (stack.Count == 0 || stack.Pop() != pairs[c])
return false;
}
}
return stack.Count == 0;
}

Follow on:

Explanation:

Use a stack to match opening and closing brackets properly.

List<string> WordBreak(string s, IList<string> wordDict) {
var wordSet = new HashSet<string>(wordDict);
var memo = new Dictionary<int, List<string>>();
return DFS(0);
List<string> DFS(int start) {
if (memo.ContainsKey(start)) return memo[start];
var res = new List<string>();
if (start == s.Length) {

res.Add("");

return res;
}
for (int end = start + 1; end <= s.Length; end++) {
string word = s.Substring(start, end - start);
if (wordSet.Contains(word)) {
foreach (var sub in DFS(end)) {
string space = sub.Length == 0 ? "" : " ";

res.Add(word + space + sub);

}
}
}
memo[start] = res;
return res;
}
}

Follow on:

int WidthOfBinaryTree(TreeNode root) {

if (root == null) return 0;

int maxWidth = 0;

Queue<(TreeNode node, int idx)> queue = new Queue<(TreeNode,

int)>();

queue.Enqueue((root, 0));

while (queue.Count > 0) {

int size = queue.Count;

int start = queue.Peek().idx;

int end = start;

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

var (node, idx) = queue.Dequeue();

end = idx;

if (node.left != null)

queue.Enqueue((node.left, 2 * idx + 1));

if (node.right != null)

queue.Enqueue((node.right, 2 * idx + 2));

maxWidth = Math.Max(maxWidth, end - start + 1);

Follow on:

return maxWidth;

Explanation:

Assign index to each node as if in a complete tree; width is max difference of indices per

level.

bool SolveSudoku(char[][] board)

{
for (int i = 0; i < 9; i++)
{
for (int j = 0; j < 9; j++)
{

Follow on:

if (board[i][j] == '.')
{
for (char c = '1'; c <= '9'; c++)
{
if (IsValid(board, i, j, c))
{
board[i][j] = c;
if (SolveSudoku(board))
return true;

else

board[i][j] = '.';
}
}
return false;
}
}
}
return true;
}

bool IsValid(char[][] board, int row, int col, char c)

{
for (int i = 0; i < 9; i++)
{
if (board[row][i] == c) return false;
if (board[i][col] == c) return false;
if (board[3 * (row / 3) + i / 3][3 * (col / 3) + i % 3] ==

c) return false;

}
return true;
}

Explanation:

Backtracking tries digits 1-9 in empty cells, validating constraints.

Answer: a / b : throw new DivideByZeroException(), _ =&gt; throw new ArgumentException("Invalid operator"), Explanation: Simple switch statement performing basic arithmetic, with divide-by-zero check.

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.

void SortColors(int[] nums)

{
int low = 0, mid = 0, high = nums.Length - 1;

while (mid <= high)

{
if (nums[mid] == 0)
(nums[low++], nums[mid++]) = (nums[mid], nums[low]);

else if (nums[mid] == 1)

mid++;

else

(nums[mid], nums[high--]) = (nums[high], nums[mid]);
}
}

Follow on:

Explanation:

Partition array into three parts in one pass using three pointers.