Question

362. Design Hit Counter

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Description

Design a hit counter which counts the number of hits received in the past 5 minutes (i.e., the past 300 seconds).

Your system should accept a timestamp parameter (in seconds granularity), and you may assume that calls are being made to the system in chronological order (i.e., timestamp is monotonically increasing). Several hits may arrive roughly at the same time.

Implement the HitCounter class:

• HitCounter() Initializes the object of the hit counter system.
• void hit(int timestamp) Records a hit that happened at timestamp (in seconds). Several hits may happen at the same timestamp.
• int getHits(int timestamp) Returns the number of hits in the past 5 minutes from timestamp (i.e., the past 300 seconds).

 

Example 1:

Input
["HitCounter", "hit", "hit", "hit", "getHits", "hit", "getHits", "getHits"]
[[], [1], [2], [3], [4], [300], [300], [301]]
Output
[null, null, null, null, 3, null, 4, 3]

Explanation
HitCounter hitCounter = new HitCounter();
hitCounter.hit(1);     // hit at timestamp 1.
hitCounter.hit(2);     // hit at timestamp 2.
hitCounter.hit(3);     // hit at timestamp 3.
hitCounter.getHits(4);   // get hits at timestamp 4, return 3.
hitCounter.hit(300);   // hit at timestamp 300.
hitCounter.getHits(300); // get hits at timestamp 300, return 4.
hitCounter.getHits(301); // get hits at timestamp 301, return 3.

 

Constraints:

• 1 <= timestamp <= 2 * 109
• All the calls are being made to the system in chronological order (i.e., timestamp is monotonically increasing).
• At most 300 calls will be made to hit and getHits.

 

Follow up: What if the number of hits per second could be huge? Does your design scale?

Solutions

Solution 1

class HitCounter:
  def __init__(self):
    """
    Initialize your data structure here.
    """
    self.counter = Counter()

  def hit(self, timestamp: int) -> None:
    """
    Record a hit.
    @param timestamp - The current timestamp (in seconds granularity).
    """
    self.counter[timestamp] += 1

  def getHits(self, timestamp: int) -> int:
    """
    Return the number of hits in the past 5 minutes.
    @param timestamp - The current timestamp (in seconds granularity).
    """
    return sum([v for t, v in self.counter.items() if t + 300 > timestamp])


# Your HitCounter object will be instantiated and called as such:
# obj = HitCounter()
# obj.hit(timestamp)
# param_2 = obj.getHits(timestamp)

class HitCounter {

  private Map<Integer, Integer> counter;

  /** Initialize your data structure here. */
  public HitCounter() {
    counter = new HashMap<>();
  }

  /**
     Record a hit.
    @param timestamp - The current timestamp (in seconds granularity).
   */
  public void hit(int timestamp) {
    counter.put(timestamp, counter.getOrDefault(timestamp, 0) + 1);
  }

  /**
     Return the number of hits in the past 5 minutes.
    @param timestamp - The current timestamp (in seconds granularity).
   */
  public int getHits(int timestamp) {
    int hits = 0;
    for (Map.Entry<Integer, Integer> entry : counter.entrySet()) {
      if (entry.getKey() + 300 > timestamp) {
        hits += entry.getValue();
      }
    }
    return hits;
  }
}

/**
 * Your HitCounter object will be instantiated and called as such:
 * HitCounter obj = new HitCounter();
 * obj.hit(timestamp);
 * int param_2 = obj.getHits(timestamp);
 */

use std::{ collections::BinaryHeap, cmp::Reverse };

struct HitCounter {
  /// A min heap
  pq: BinaryHeap<Reverse<i32>>,
}

impl HitCounter {
  fn new() -> Self {
    Self {
      pq: BinaryHeap::new(),
    }
  }

  fn hit(&mut self, timestamp: i32) {
    self.pq.push(Reverse(timestamp));
  }

  fn get_hits(&mut self, timestamp: i32) -> i32 {
    while let Some(Reverse(min_elem)) = self.pq.peek() {
      if *min_elem <= timestamp - 300 {
        self.pq.pop();
      } else {
        break;
      }
    }

    self.pq.len() as i32
  }
}