Technical Assessment

🛠 1. Two Sum Problem (HashMap)
Write a method that takes an integer array and a target sum. Return the indices of the two numbers that add up to the target.
📌 Use a Dictionary (HashMap) for an optimal solution.

csharp

public int[] TwoSum(int[] nums, int target) {
    Dictionary<int, int> numMap = new Dictionary<int, int>();
    for (int i = 0; i < nums.Length; i++) {
        int diff = target - nums[i];
        if (numMap.ContainsKey(diff)) {
            return new int[] { numMap[diff], i };
        }
        numMap[nums[i]] = i;
    }
    return new int[0]; // No solution
}

🔹 Practice: Try implementing this using nested loops (brute force) and compare performance.

📚 2. Reverse a Linked List
Write a function to reverse a singly linked list.
📌 Understand how pointers work in linked lists.

csharp

public ListNode ReverseList(ListNode head) {
    ListNode prev = null, curr = head;
    while (curr != null) {
        ListNode next = curr.next;
        curr.next = prev;
        prev = curr;
        curr = next;
    }
    return prev;
}

🔹 Practice: Implement recursively as well.

🔍 3. Find the First Non-Repeating Character in a String
Write a method that returns the first character in a string that does not repeat.
📌 Use a dictionary for frequency counting.

csharp

public char FirstNonRepeatingChar(string s) {
    Dictionary<char, int> count = new Dictionary<char, int>();
    foreach (char c in s) count[c] = count.GetValueOrDefault(c, 0) + 1;
    foreach (char c in s) if (count[c] == 1) return c;
    return '_'; // No non-repeating character
}

🔹 Practice: Handle uppercase/lowercase sensitivity.

📈 4. FizzBuzz (Classic)
Print numbers from 1 to n.

If divisible by 3, print "Fizz".

If divisible by 5, print "Buzz".

If divisible by both, print "FizzBuzz".

csharp

public void FizzBuzz(int n) {
    for (int i = 1; i <= n; i++) {
        if (i % 3 == 0 && i % 5 == 0) Console.WriteLine("FizzBuzz");
        else if (i % 3 == 0) Console.WriteLine("Fizz");
        else if (i % 5 == 0) Console.WriteLine("Buzz");
        else Console.WriteLine(i);
    }
}

🔹 Practice: Solve this using a switch expression in C#.

💾 5. Implement a Stack Using Two Queues
Implement a stack (LIFO) using only two queues (FIFO).
📌 Understand how stacks & queues differ.

csharp

using System.Collections.Generic;

public class MyStack {
    private Queue<int> q1 = new Queue<int>();
    private Queue<int> q2 = new Queue<int>();

    public void Push(int x) {
        q2.Enqueue(x);
        while (q1.Count > 0) q2.Enqueue(q1.Dequeue());
        var temp = q1;
        q1 = q2;
        q2 = temp;
    }

    public int Pop() => q1.Dequeue();
    public int Top() => q1.Peek();
    public bool Empty() => q1.Count == 0;
}

🔹 Practice: Implement using a single queue.

🌳 6. Binary Search Tree: Find Depth
Write a function to find the depth of a binary tree.

csharp

public int MaxDepth(TreeNode root) {
    if (root == null) return 0;
    return 1 + Math.Max(MaxDepth(root.left), MaxDepth(root.right));
}

🔹 Practice: Implement an iterative BFS solution.

🧠 7. OOP: Design a Parking System
Design a Parking System with the following classes:

ParkingSystem:

AddCar(int carType): Checks if there's space for a car.

Uses big, medium, and small slots.

csharp

public class ParkingSystem {
    private int[] slots;

    public ParkingSystem(int big, int medium, int small) {
        slots = new int[] { big, medium, small };
    }

    public bool AddCar(int carType) {
        if (slots[carType - 1] > 0) {
            slots[carType - 1]--;
            return true;
        }
        return false;
    }
}

🔹 Practice: Extend with dynamic pricing.

⚡ 8. Implement an LRU Cache
Implement a Least Recently Used (LRU) Cache using a Dictionary + LinkedList.

csharp

using System.Collections.Generic;

public class LRUCache {
    private int capacity;
    private Dictionary<int, LinkedListNode<(int key, int value)>> cache;
    private LinkedList<(int key, int value)> order;

    public LRUCache(int capacity) {
        this.capacity = capacity;
        cache = new Dictionary<int, LinkedListNode<(int key, int value)>>();
        order = new LinkedList<(int key, int value)>();
    }

    public int Get(int key) {
        if (!cache.ContainsKey(key)) return -1;
        var node = cache[key];
        order.Remove(node);
        order.AddFirst(node);
        return node.Value.value;
    }

    public void Put(int key, int value) {
        if (cache.ContainsKey(key)) order.Remove(cache[key]);
        if (cache.Count == capacity) {
            cache.Remove(order.Last.Value.key);
            order.RemoveLast();
        }
        order.AddFirst((key, value));
        cache[key] = order.First;
    }
}

🔹 Practice: Extend it with time-based expiry.

🔢 9. Count Distinct Elements in an Array
Count the number of distinct elements in an array. 📌 Use a HashSet for efficiency.

csharp

public int CountDistinct(int[] nums) {
    return new HashSet<int>(nums).Count;
}

🔹 Practice: Do it without HashSet.

🔄 10. Rotate an Array
Write a function to rotate an array k times to the right.
📌 Use an efficient approach (O(n) time, O(1) space).

csharp

public void Rotate(int[] nums, int k) {
    k %= nums.Length;
    Array.Reverse(nums, 0, nums.Length);
    Array.Reverse(nums, 0, k);
    Array.Reverse(nums, k, nums.Length - k);
}

🔹 Practice: Solve without modifying the original array.

Next Steps
🔹 Time yourself while solving these.
🔹 Use unit tests to verify your solutions.
🔹 Try solving in multiple ways (brute force, optimized).