Reading into collection and retrieving top 5

Question

I was asked this recently in an interview. Application that is reading ticker and trade volume from a live feed, E.g. AAPL 1000, TWTR 500, MSFT 500, AAPL 500 ... So, AAPL

Answer 1

Assuming ticker symbol and trade volume are stored together in an instance of some class TickerAndTradeVolume, you could have references to objects contained in multiple data structures.

So a hash map may have ticker symbol as key and TickerAndTradeVolume as value. Then references to TickerAndTradeVolume instances can also be stored in a priority queue. Instances are re-inserted into the PQ on every volume update.

The top n by volume are always available in log(n) amortized time complexity to maintain priorities by trade volume, which is asymptotically faster than sorting via Treemap time and again.

Something like this

    Map<String, TickerAndTradeVolume> feed;
    PriorityQueue<TickerAndTradeVolume> pq;

    class TickerAndTradeVolume implements Comparable<TickerAndTradeVolume> {
        private String ticker;
        private double volume;

        TickerAndTradeVolume(String ticker, double volume) {
            this.ticker = ticker;
            this.volume = volume;
        }

        void increaseVolumeBy(double volume) {
            this.volume += volume;
        }

        @Override
        public int compareTo(TickerAndTradeVolume that) {
            return (int) (that.volume - this.volume);
        }

        @Override
        public boolean equals(Object obj) {
            if (this == obj) {
                return true;
            }
            if(obj instanceof String) {
                TickerAndTradeVolume that = (TickerAndTradeVolume) obj;
                return this.ticker.equals(that.ticker);
            }
            return false;
        }
    }

    void addOrUpdateStockVolume(TickerAndTradeVolume tt) {
        if(!feed.containsKey(tt.ticker)) {
            feed.put(tt.ticker, tt);
            pq.add(tt);
        }
        else {
            feed.get(tt.ticker).increaseVolumeBy(tt.volume);
            // take out and put back in to trigger heap operations
            pq.remove(feed.get(tt.ticker));
            pq.add(feed.get(tt.ticker));
        }
    }

    List<TickerAndTradeVolume> getTopMaxFive() {
        List<TickerAndTradeVolume> topFive = new ArrayList<TickerAndTradeVolume>(5);
        int pqSize = pq.size();
        for(int i = 0; i < 5 && i < pqSize; i++) {
            // poll() takes from the top of the heap
            topFive.add(pq.poll());
        }
        for(TickerAndTradeVolume tt : topFive) {
            // put back what we took out
            pq.add(tt);
        }
        return topFive;
    }