Interview Questions

DP solves problems by breaking them into overlapping subproblems and storing results (memoization or tabulation) to avoid recomputation. Use it when a problem has optimal substructure and overlapping subproblems — e.g. knapsack, edit distance, longest common subsequence.

JS runs on a single thread with a call stack. Async callbacks go to task queues (macrotasks like setTimeout, microtasks like Promises). The event loop pushes queued callbacks onto the stack only when it is empty, draining all microtasks before the next macrotask.

A deadlock is when processes wait on each other indefinitely. It needs four conditions simultaneously (Coffman): mutual exclusion, hold-and-wait, no preemption, and circular wait. Breaking any one prevents deadlock.

The Global Interpreter Lock allows only one thread to execute Python bytecode at a time, simplifying memory management but limiting CPU-bound multithreading. Use multiprocessing or native extensions for CPU parallelism; threads still help with I/O-bound work.

Generate a unique short code (base62 of an auto-increment ID or a hash) mapping to the long URL in a key-value/relational store. Add caching (Redis) for hot reads, a redirect service returning 301/302, analytics, and consider sharding + a counter service for scale.

In a distributed system you can guarantee at most two of Consistency, Availability, and Partition tolerance. Since network partitions are unavoidable, you effectively choose between consistency (CP) and availability (AP) during a partition.