How long this price has held up
A trading widget shows, for each day, how many days back the price has been no higher than it is today — today included.
- Count today and then each earlier consecutive day whose price is less than or equal to today’s.
- Stop at the first earlier day priced above today.
- The first day always answers 1.
priceRun(prices: list<int>) → list<int>
Java needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
List<Integer> priceRun(List<Integer> prices) {
}Worked examples
| Call | Result |
|---|---|
priceRun(Main.<Integer>ls(100, 80, 60, 70, 60, 75, 85)) | Main.<Integer>ls(1, 1, 1, 2, 1, 4, 6) |
priceRun(Main.<Integer>ls(10, 20, 30)) | Main.<Integer>ls(1, 2, 3) |
priceRun(Main.<Integer>ls(30, 20, 10)) | Main.<Integer>ls(1, 1, 1) |
priceRun(Main.<Integer>ls(5, 5, 5)) | Main.<Integer>ls(1, 2, 3) |
Hint
Rather than walking backwards each day, keep a stack of earlier days that were priced higher. Popping the ones that were not gives you the run in one pass.
Reference solution in Java
List<Integer> priceRun(List<Integer> prices) {
List<Integer> runs = new ArrayList<>();
Deque<Integer> higher = new ArrayDeque<>();
for (int i = 0; i < prices.size(); i++) {
while (!higher.isEmpty() && prices.get(higher.peek()) <= prices.get(i)) higher.pop();
runs.add(higher.isEmpty() ? i + 1 : i - higher.peek());
higher.push(i);
}
return runs;
}