Microsoft

The interviewer started by asking me to find the length of the longest valid parentheses substring in an expression string; I initially walked through a stack-based approach running in O(N) time and O(N) space, but the interviewer asked if I could optimize the memory footprint, so I followed up with a two-pass left-to-right and right-to-left counter method that brought space complexity down to O(1). I was presented with an unsorted log file containing millions of user transaction IDs and asked to find the k most frequent transactions in real time; after establishing a baseline using a hash map combined with a min-heap of size k, the interviewer asked how the system would scale if data arrived distributed across multiple worker nodes, leading into a discussion on MapReduce partitioning and Count-Min Sketch. The interviewer framed a problem around a ride-sharing fleet where drivers and passenger pickups are modeled as bipartite vertices on a grid with fixed travel costs; they asked me to compute the minimum overall trip assignment cost, which I modeled as a maximum-weight bipartite matching problem and solved step-by-step using the Hungarian algorithm while explaining time and state constraints.