Software Engineer · Phone Screen · Coding

Uber — Software Engineer ✅ Passed

Level: Mid-Level

Round: Phone Screen · Type: Coding · Difficulty: 7/10 · Duration: 60 min · Interviewer: Unfriendly

Topics: Data Structures, Hash Table, System Design, Concurrency

Location: San Francisco, CA

Interview date: 2025-12-31

Summary

I had a technical phone interview where I was asked to implement a Counter class. The elements inserted into the counter have an expiration time set during initialization.

The class needs to implement three functions:

• put(element): Adds an element to the counter.
• get_count(element): Returns the current valid count of the element in the counter.
• get_total_count(): Returns the current count of all valid elements in the counter.

For example: counter(window=10) time 1: counter.put(‘a’) time 3: counter.put(‘a’) time 5: counter.put(‘b’) time 6: counter.get_count(‘a’) -> 2 time 6: counter.get_total_count() -> 3 time 12: counter.get_count(‘a’) -> 1 time 12: counter.get_total_count() -> 2

Details

The coding question I got was to implement a Counter class with expiration:

`python class Counter: def init(self, window): self.window = window self.data = {}

def put(self, element):
    current_time = time.time()
    if element in self.data:
        self.data[element].append(current_time)
    else:
        self.data[element] = [current_time]

def get_count(self, element):
    current_time = time.time()
    if element not in self.data:
        return 0
    
    valid_times = [t for t in self.data[element] if current_time - t <= self.window]
    return len(valid_times)

def get_total_count(self):
    current_time = time.time()
    total_count = 0
    for element in self.data:
        valid_times = [t for t in self.data[element] if current_time - t <= self.window]
        total_count += len(valid_times)
    return total_count

`

My approach:

1. I used a dictionary to store elements and their timestamps.
2. The put function adds the element with the current timestamp.
3. The get_count function filters the timestamps within the window.
4. The get_total_count function iterates through all elements and sums their valid counts.

Follow-up: The follow-up question asked about handling a million inputs per second. I suggested using batch aggregation to update the state every second, employing a queue with lazy deletion.