Want to try solving this problem? You can submit your code online if you log in or register.

Roderick was once the world's greatest light-switch switcher. With astonishing dexterity and lightning speed, Rod's unique capability of hitting many adjacent light-switches simultaneously gave him an edge unparalleled in the light-switching business. However in these tough economic times, Rod has had to turn to alternate pursuits. Yesterday he was called up and offered a place on the popular competitive cooking TV show Master Chef, but he needs your help to determine whether he has what it takes to truly master the kitchen.

Roderick will be given N ingredients lined up in front of him. His strategy
for speedy cooking involves selecting, at random, a set of ingredients that
form one contiguous subsequence of the line-up (that is, all ingredients
between the a^{th} and b^{th} ingredient
inclusive for some random selection of
a and b such that 1 ≤ a ≤ b ≤ N). In one swift swoop using one
of his exceptionally large hands, Roderick will collect all the selected
ingredients and pour them into the pot for mixing. Each ingredient has a
tastiness value T (which may be negative - some ingredients are truly foul),
and the tastiness of the final meal will be the average tastiness of all
selected ingredients. For example, in the following line-up of ingredients, if Roderick
happens to select the ones with tastiness 3, -1 and 5, the tastiness
of the meal would be (3 + -1 + 5) / 3 = 2 13.

6 | 3 | -1 | 5 |

Since Roderick has no idea in advance which subsequence of ingredients he will
happen to pick up on the day of the competition, he wants you to write a
program that will calculate the expected tastiness of the meal he will create
(ie. average tastiness of all possible meals he may create). In the example
above with 4 ingredients, there are 10 possible selections of ingredients that
Roderick could make meals from. The meal tastiness values would be: 6, 3, -1,
5, 4.5, 1, 2, 2.6 ^{1},
2.3 and 3.25. The average
tastiness of all these 10 possible meals is 2.875. As Roderick isn't great
with numbers, he wants you to give the final expected tastiness rounded
to the next integer towards zero (that is, truncated to zero decimal places).
In this example, the answer would hence be 2.

- The first line of input will contain one integer N, representing the
number of ingredients.
- The following line will contain N integers, representing the tastiness
of each ingredient from left to right.

Output should consist of a single integer: the (rounded) average tastiness of all possible meals, where a meal's tastiness is the average tastiness of all ingredients in a subsequence of ingredients. Rounding is towards zero.

4 6 3 -1 5

2

8 -3 10 19 9 7 10 0 5

8

3 -10 -4 1

-4

The first sample case corresponds to the example discussed in the problem. In the second sample case, the average tastiness of the 36 possible meals is approximately 8.12, which is truncated to 8. In the third sample case, the average is -4.30556, which is rounded towards zero to become -4.

In all subtasks, the tastiness of each ingredient will be an integer between -1000 and 1000 inclusive.

- Subtask 1 (20 points): 1 ≤ N ≤ 200
- Subtask 2 (20 points): 1 ≤ N ≤ 2,000
- Subtask 3 (60 points): 1 ≤ N ≤ 200,000

You will score the full allocated marks for a subtask if your program returns the correct answer for every test case within that subtask. If your program fails any test case in a subtask, your score will be 0 for that subtask.

Privacy
statement

© Australian Mathematics Trust 2001-2022

Page generated: 25 September 2022, 2:05am AEST