Hit Counter - Amazon Top Interview Questions

Problem Statement :

Implement a hit counter which keeps track of number of the number of hits in the last 60 seconds.

add(int timestamp) which adds timestamp in seconds in the hit counter
count(int timestamp) which returns the number of hits that have been made in the last 60 seconds, given the current time is timestamp.
You can assume that the timestamps passed into add and count are monotonically increasing.


n ≤ 100,000 where n is the number of calls that are made to add and count

Example 1


methods = ["constructor", "add", "add", "count", "add", "count"]
arguments = [[], [10], [40], [40], [70], [100]]`


[None, None, None, 2, None, 2]


We create a HitCounter
We add timestamp 10 to the data structure
We add timestamp 40 to the data structure
We count the number of timestamps that are within last 60 seconds of 40. There's 10 and 40 so we 
return 2
We add timestamp 70 to the data structure
We count the number of timestamps that are within last 60 seconds of 100. There's 40 and 70 so we return 2

Solution :


                        Solution in C++ :

class HitCounter {
    vector<int> v;
    HitCounter() {

    void add(int timestamp) {

    int count(int timestamp) {
        return v.end() - lower_bound(v.begin(), v.end(), timestamp - 60);

                        Solution in Python : 
class HitCounter:
    def __init__(self):
        self.q = deque()

    def _cleanup(self, timestamp):
        while self.q and timestamp - self.q[0] > 60:

    def add(self, timestamp):

    def count(self, timestamp):
        return len(self.q)

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