C# Coding Interview

The collections that matter, what each call costs, and the traps that turn O(n) into O(n²).

Read the input size as a hint. It tells you the complexity the interviewer expects.

Seven phases, a time budget, what to say when stuck, and a worked transcript.

Dry runs, five boundary checks, edge cases by input type, and quick tests in C#.

LINQ for shaping data, streaming big files, hand-coded statistics, sampling and SIMD.

The base class library calls you use daily, grouped by job, with the traps.

A contiguous run in an array or string, and you want the longest, shortest or best one.

• Maximum Subarray of Size K
• Longest Substring Without Repeating Characters
• Minimum Size Subarray Sum
• Minimum Window Substring

A sorted array, and you need a pair, a triplet, or an in-place compaction.

• Two Sum II — Input Array Is Sorted
• Remove Duplicates from Sorted Array
• 3Sum
• Container With Most Water

A linked list or an implicit sequence, and you suspect a cycle or need the midpoint.

• Linked List Cycle
• Middle of the Linked List
• Happy Number
• Find the Duplicate Number

Time ranges or numeric ranges that overlap, and you need them merged, counted or trimmed.

• Merge Intervals
• Insert Interval
• Non-overlapping Intervals
• Meeting Rooms II

Numbers drawn from a bounded range, and you must find what is missing or duplicated in O(1) space.

• Missing Number
• Find All Numbers Disappeared in an Array
• Find the Duplicate Number
• First Missing Positive

Pointer surgery on a linked list with no extra memory allowed.

• Reverse Linked List
• Reverse Linked List II
• Reverse Nodes in k-Group

Level-by-level processing, or the shortest path in an unweighted graph.

• Binary Tree Level Order Traversal
• Binary Tree Zigzag Level Order Traversal
• Rotting Oranges
• Word Ladder

Explore every path, compute something bottom-up, or flood-fill a region.

• Maximum Depth of Binary Tree
• Path Sum II
• Number of Islands
• Clone Graph

The K best, worst or most frequent items, from a large or streaming dataset.

• Kth Largest Element in an Array
• Top K Frequent Elements
• K Closest Points to Origin

Enumerate every combination, permutation or arrangement, pruning the dead ends.

• Subsets
• Permutations
• Combination Sum
• N-Queens

Anything with a monotone yes/no boundary, sorted or rotated, in O(log n).

• Binary Search
• Search in Rotated Sorted Array
• Find First and Last Position of Element in Sorted Array
• Peak Index in a Mountain Array

An optimal value or a count of ways, built from overlapping subproblems.

• Climbing Stairs
• Coin Change
• Longest Increasing Subsequence
• 0/1 Knapsack
• Maximum Subarray (Kadane)
• Longest Common Subsequence
• Edit Distance

Range sums where a sliding window fails: negative values, exact targets, or divisibility.

• Subarray Sum Equals K
• Continuous Subarray Sum
• Product of Array Except Self
• Subarray Sums Divisible by K

For each element, the nearest larger or smaller one, and how far it reaches.

• Daily Temperatures
• Next Greater Element II
• Trapping Rain Water
• Largest Rectangle in Histogram

Ordering under dependencies, and detecting when no valid order exists.

• Course Schedule
• Course Schedule II
• Alien Dictionary

Connectivity on a graph whose edges keep arriving, where re-running DFS is too slow.

• Number of Connected Components
• Redundant Connection
• Accounts Merge
• Number of Islands II

One pass, one local choice at each step, and an exchange argument that proves it safe.

• Jump Game
• Gas Station
• Partition Labels
• Task Scheduler

Many words, shared prefixes, and lookups by prefix or with wildcards.

• Implement Trie
• Design Add and Search Words
• Word Search II
• Replace Words

XOR cancels pairs, x & (x - 1) drops a bit, and an int can act as a set.

• Single Number
• Number of 1 Bits
• Counting Bits
• Sum of Two Integers

Merge K sorted inputs, or keep a running median with a heap on each side.

• Merge k Sorted Lists
• Kth Smallest Element in a Sorted Matrix
• Find Median from Data Stream
• Smallest Range Covering Elements from K Lists

Shortest path when edges cost different amounts: Dijkstra, 0-1 BFS, Bellman-Ford.

• Network Delay Time
• Path With Minimum Effort
• Cheapest Flights Within K Stops
• Swim in Rising Water

The best value in a moving window, in O(n) total.

• Sliding Window Maximum
• Shortest Subarray with Sum at Least K
• Constrained Subsequence Sum
• Longest Continuous Subarray With Absolute Diff Limit

Build a class where every operation is O(1) by pairing two structures.

• LRU Cache
• Min Stack
• Insert Delete GetRandom O(1)
• LFU Cache

Value vs reference, boxing, ref and in, ref struct, Span<T>, stackalloc, inline arrays.

Constraints, variance, static abstract members, generic math and generic attributes.

Closures and capture, static lambdas, local functions, events, function pointers.

Deferred execution, yield state machines, custom operators, IQueryable and expression trees.

Task vs ValueTask, deadlocks, cancellation, IAsyncEnumerable, Channels and WhenEach.

Every pattern kind, switch expressions, records, with, required and primary constructors.

Nullable reference types, null operators, checked math and exceptions done well.

The GC, dispose, pools, spans, unsafe code, frozen collections and measuring.

Reflection, attributes, expression trees, source generators, partial members, field.

Locks and the new Lock type, Interlocked, concurrent collections, PLINQ, classic tasks.

Every language version, newest first, with a full section on each C# 14 feature.