Stock Price Tracker

A trading feed sends (timestamp, price) records for one stock. Records can arrive out of order, and a record for a timestamp that was already seen is a correction: it replaces the earlier price for that timestamp. Build a StockTracker class:

• StockTracker() starts with no records.
• update(timestamp, price) records the price at timestamp, or corrects it.
• current() returns the price at the latest timestamp seen so far.
• highest() returns the highest price over all timestamps, using the corrected prices.
• lowest() returns the lowest price over all timestamps, using the corrected prices.

current, highest and lowest are only called after at least one update.

Examples

st = StockTracker()
st.update(1, 10)
st.update(2, 5)
st.current()     # returns 5
st.highest()     # returns 10
st.update(1, 3)  # the price at time 1 was wrong
st.highest()     # returns 5
st.lowest()      # returns 3
st.update(4, 2)
st.current()     # returns 2

Constraints

• Up to 150,000 calls; timestamps and prices are whole numbers from 1 to 10^9
• With 50,000 timestamps and tens of thousands of questions, looking at every timestamp for each highest or lowest is far too slow.