362. Design Hit Counter

/**

 * 362. Design Hit Counter

 * Design a hit counter which counts the number of hits received in the past 5 minutes.

 * Each function accepts a timestamp parameter (in seconds granularity) and you may assume that calls are

 * being made to the system in chronological order (ie, the timestamp is monotonically increasing).

 * You may assume that the earliest timestamp starts at 1.

 * It is possible that several hits arrive roughly at the same time.

 * Example:

 * HitCounter counter = new HitCounter();

 * // hit at timestamp 1.

 * counter.hit(1);

 * // hit at timestamp 2.

 * counter.hit(2);

 * // hit at timestamp 3.

 * counter.hit(3);

 * // get hits at timestamp 4, should return 3.

 * counter.getHits(4);

 * // hit at timestamp 300.

 * counter.hit(300);

 * // get hits at timestamp 300, should return 4.

 * counter.getHits(300);

 * // get hits at timestamp 301, should return 3.

 * counter.getHits(301);

 * Follow up:

 * What if the number of hits per second could be very large? Does your design scale?

 */

See the solution to the problem on github here: https://github.com/zcoderz/leetcode/blob/main/src/main/java/frequent/medium/HitCounter.java

Solution is simple:

  1. Keep the count of times in a treemap
  2. For insertions, add the data to time stamp and increment count
  3. For retrieval
    1. Use headMap to get values less than and inclusive of the time stamp
    1. Use tailMap to filter out values less than the prior (timestamp – 5*60) threshold
    1. Iterate over data to calculate total count while skipping the row with key equal to prior

Here is the code:

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/** Record a hit.
 @param timestamp - The current timestamp (in seconds granularity). */
void hit(int timestamp) {
    int count = set.getOrDefault(timestamp, 0);
    set.put(timestamp, count+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 prior = timestamp - (60*5);
    //search the lower and higher edges to find the count.
    SortedMap<Integer, Integer> less = set.headMap(timestamp, true);
    SortedMap<Integer, Integer> tailMap =  less.tailMap(prior);
    int count = 0;
    for (Map.Entry<Integer, Integer> entry : tailMap.entrySet()) {
        //strip out any record equal to prior value as that maybe present but shouldn't to be included
        if (entry.getKey().equals(prior)) continue;
        count += entry.getValue();
    }
    return count;
}