To find the middle of a singly linked list in a single pass, we use the two-pointer approach (often called the tortoise and hare algorithm). We initialize two pointers, slow and fast, at the head of the list. We traverse the list, moving the fast pointer by two steps and the slow pointer by one step at each iteration. When the fast pointer reaches the end of the list (or its next node is null), the slow pointer will point to the exact middle node. Here is the Java implementation: ```java public Node findMiddle(Node head) { if (head == null) return null; Node slow = head; Node fast = head; while (fast != null && fast.next != null) { slow = slow.next; fast = fast.next.next; } return slow; } ```

ACID properties ensure that database transactions are processed reliably: 1. Atomicity: Guarantees that all operations within a transaction are completed successfully; if any operation fails, the entire transaction is rolled back, leaving the database unchanged. In banking, this prevents scenarios where money is debited from one account but not credited to the other. 2. Consistency: Ensures that a transaction can only bring the database from one valid state to another, maintaining database constraints. For instance, the total sum of money across two accounts must remain the same before and after a transfer. 3. Isolation: Ensures that concurrent transactions execute independently of one another without interference. This prevents anomalies like dirty reads or non-repeatable reads during simultaneous transfers. 4. Durability: Guarantees that once a transaction commits, its changes survive system failures, stored permanently in non-volatile memory.

Optimistic Locking assumes that multiple transactions can complete without affecting each other. It verifies before committing if another transaction has modified the data (usually via a version number or timestamp). If data has changed, the transaction is rolled back and retried. It is highly efficient for read-heavy operations with low conflict rates. Pessimistic Locking assumes conflicts are likely. It locks the resource (using commands like SELECT FOR UPDATE) when the transaction starts, preventing other transactions from modifying or reading it until the lock is released. It is preferred for write-heavy systems where transaction failure must be minimized (like balance debits).

Thread safety means that a class or method can be executed concurrently by multiple threads without causing race conditions or data inconsistency. It can be achieved using: 1. Synchronized Keyword: Locks the object or class to serialize thread access. 2. ReentrantLock class: Offers advanced locking features like tryLock() and interruptible locks. 3. Volatile Keyword: Ensures changes to a variable are immediately visible to all threads. 4. Atomic classes: Classes like AtomicInteger use low-level compare-and-swap (CAS) operations. 5. Concurrent Collections: Classes like ConcurrentHashMap allow safe multi-threaded manipulation without locking the entire structure.

Garbage Collection (GC) in Java is an automatic memory management process that identifies and deletes unused objects in the heap. The heap is divided into Generations: Young Generation (Eden space, Survivor spaces S0/S1) where new objects are created, and Old Generation where long-lived objects are promoted. GC algorithms include: 1. Serial GC: Single-threaded collector, pauses application threads (Stop-The-World). 2. Parallel GC: Multi-threaded collector for Young/Old generations, optimized for throughput. 3. G1 (Garbage-First) GC: Divides the heap into regions, performs concurrent marking, and cleans regions with the most garbage first. Default in modern Java. 4. ZGC: Ultra-low latency collector designed for large heaps, with pause times under a millisecond.

To retrieve the highest salary for each department, we use the GROUP BY clause along with the MAX aggregate function. The SQL query is: ```sql SELECT department_id, MAX(salary) AS highest_salary FROM Employee GROUP BY department_id; ``` If we also need to retrieve the details of the employee earning that salary, we can use a subquery or a CTE with the `ROW_NUMBER()` or `DENSE_RANK()` window function: ```sql WITH RankedEmployees AS ( SELECT employee_id, name, department_id, salary, DENSE_RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) as rank FROM Employee ) SELECT department_id, employee_id, name, salary FROM RankedEmployees WHERE rank = 1; ```

A deadlock is a situation where two or more threads are blocked forever, each waiting for a lock held by the other. The four necessary Coffman conditions are: 1. Mutual Exclusion: At least one resource must be held in a non-shareable mode. 2. Hold and Wait: A thread must be holding at least one resource and waiting to acquire additional resources held by other threads. 3. No Preemption: Resources cannot be forcibly taken from a thread holding them. 4. Circular Wait: A closed chain of threads exists, where each thread waits for a resource held by the next thread in the chain.

Monolithic architecture builds an entire application as a single, indivisible unit. It is simple to deploy and test initially, but scaling, updating, or modifying specific components requires redeploying the entire codebase. A single module failure can crash the whole system. Microservices architecture structures an application as a collection of small, loosely coupled services. Each service represents a specific business capability, can be developed and deployed independently, and communicates via lightweight protocols (REST or gRPC). This facilitates independent scaling, fault isolation, and technology diversity, but introduces complexity in deployment, distributed transaction management, and network communication.