
USACO 2025 February Contest, Platinum
Problem 1. Min Max Subarrays
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**Note: The time limit for this problem is 3s, 1.5x the default.**
You are given a length-$N$ integer array $a_1,a_2,\dots,a_N$ ($2\le N\le 10^6, 1\le a_i\le N$). Output the sum of the answers for the subproblem below over all $N(N+1)/2$ contiguous subarrays of $a$.
Given a nonempty list of integers, alternate the following operations (starting with the first operation) until the list has size exactly one.
- Replace two consecutive integers in the list with their minimum.
- Replace two consecutive integers in the list with their maximum.
Determine the maximum possible value of the final remaining integer.
For example,
[4, 10, 3] -> [4, 3] -> [4] [3, 4, 10] -> [3, 10] -> [10]
In the first array, $(10, 3)$ is replaced by $\min(10, 3)=3$ and $(4, 3)$ is replaced by $\max(4, 3)=4$.
INPUT FORMAT (input arrives from the terminal / stdin):
The first line contains $N$.The second line contains $a_1,a_2,\dots,a_N$.
OUTPUT FORMAT (print output to the terminal / stdout):
The sum of the answer to the subproblem over all subarrays.SAMPLE INPUT:
2 2 1
SAMPLE OUTPUT:
4
The answer for $[2]$ is $2$, the answer for $[1]$ is $1$, and the answer for $[2, 1]$ is $1$.
Thus, our output should be $2+1+1 = 4$.
SAMPLE INPUT:
3 3 1 3
SAMPLE OUTPUT:
12
SAMPLE INPUT:
4 2 4 1 3
SAMPLE OUTPUT:
22
Consider the subarray $[2, 4, 1, 3]$.
- Applying the first operation on (1, 3), our new array is $[2, 4, 1]$.
- Applying the second operation on (4, 1), our new array is $[2, 4]$.
- Applying the third operation on (2, 4), our final number is $2$.
It can be proven that $2$ is the maximum possible value of the final number.
SCORING:
- Inputs 4-5: $N\le 100$
- Inputs 6-7: $N\le 5000$
- Inputs 8-9: $\max(a)\le 10$
- Inputs 10-13: No additional constraints.
Problem credits: Benjamin Qi