Data Structures & Algorithms for C# Interviews

Essential data structures and algorithms with C# implementations for technical interviews.

Arrays and Strings

Common Patterns

• Two Pointers
• Sliding Window
• Hash Maps

// Two Sum Problem
public int[] TwoSum(int[] nums, int target)
{
    var map = new Dictionary<int, int>();
    
    for (int i = 0; i < nums.Length; i++)
    {
        int complement = target - nums[i];
        if (map.ContainsKey(complement))
            return new[] { map[complement], i };
        map[nums[i]] = i;
    }
    
    return Array.Empty<int>();
}

Lists and LinkedLists

Singly Linked List

public class ListNode
{
    public int Val { get; set; }
    public ListNode Next { get; set; }
}

// Reverse Linked List
public ListNode ReverseList(ListNode head)
{
    ListNode prev = null;
    ListNode current = head;
    
    while (current != null)
    {
        ListNode next = current.Next;
        current.Next = prev;
        prev = current;
        current = next;
    }
    
    return prev;
}

Trees and Graphs

Binary Tree Traversal

public class TreeNode
{
    public int Val { get; set; }
    public TreeNode Left { get; set; }
    public TreeNode Right { get; set; }
}

// Inorder Traversal (Recursive)
public void InorderTraversal(TreeNode root, List<int> result)
{
    if (root == null) return;
    
    InorderTraversal(root.Left, result);
    result.Add(root.Val);
    InorderTraversal(root.Right, result);
}

Sorting and Searching

Quick Sort

Time: O(n log n) average, O(n²) worst Space: O(log n)

Binary Search

public int BinarySearch(int[] arr, int target)
{
    int left = 0, right = arr.Length - 1;
    
    while (left <= right)
    {
        int mid = left + (right - left) / 2;
        
        if (arr[mid] == target) return mid;
        if (arr[mid] < target) left = mid + 1;
        else right = mid - 1;
    }
    
    return -1;
}

Dynamic Programming

Fibonacci (Memoization)

private Dictionary<int, long> memo = new();

public long Fibonacci(int n)
{
    if (n <= 1) return n;
    if (memo.ContainsKey(n)) return memo[n];
    
    memo[n] = Fibonacci(n - 1) + Fibonacci(n - 2);
    return memo[n];
}

Big O Notation

Related Topics

• System Design
• Testing Patterns