codeforces#P1385C. Make It Good
Make It Good
Description
You are given an array $a$ consisting of $n$ integers. You have to find the length of the smallest (shortest) prefix of elements you need to erase from $a$ to make it a good array. Recall that the prefix of the array $a=[a_1, a_2, \dots, a_n]$ is a subarray consisting several first elements: the prefix of the array $a$ of length $k$ is the array $[a_1, a_2, \dots, a_k]$ ($0 \le k \le n$).
The array $b$ of length $m$ is called good, if you can obtain a non-decreasing array $c$ ($c_1 \le c_2 \le \dots \le c_{m}$) from it, repeating the following operation $m$ times (initially, $c$ is empty):
- select either the first or the last element of $b$, remove it from $b$, and append it to the end of the array $c$.
For example, if we do $4$ operations: take $b_1$, then $b_{m}$, then $b_{m-1}$ and at last $b_2$, then $b$ becomes $[b_3, b_4, \dots, b_{m-3}]$ and $c =[b_1, b_{m}, b_{m-1}, b_2]$.
Consider the following example: $b = [1, 2, 3, 4, 4, 2, 1]$. This array is good because we can obtain non-decreasing array $c$ from it by the following sequence of operations:
- take the first element of $b$, so $b = [2, 3, 4, 4, 2, 1]$, $c = [1]$;
- take the last element of $b$, so $b = [2, 3, 4, 4, 2]$, $c = [1, 1]$;
- take the last element of $b$, so $b = [2, 3, 4, 4]$, $c = [1, 1, 2]$;
- take the first element of $b$, so $b = [3, 4, 4]$, $c = [1, 1, 2, 2]$;
- take the first element of $b$, so $b = [4, 4]$, $c = [1, 1, 2, 2, 3]$;
- take the last element of $b$, so $b = [4]$, $c = [1, 1, 2, 2, 3, 4]$;
- take the only element of $b$, so $b = []$, $c = [1, 1, 2, 2, 3, 4, 4]$ — $c$ is non-decreasing.
Note that the array consisting of one element is good.
Print the length of the shortest prefix of $a$ to delete (erase), to make $a$ to be a good array. Note that the required length can be $0$.
You have to answer $t$ independent test cases.
The first line of the input contains one integer $t$ ($1 \le t \le 2 \cdot 10^4$) — the number of test cases. Then $t$ test cases follow.
The first line of the test case contains one integer $n$ ($1 \le n \le 2 \cdot 10^5$) — the length of $a$. The second line of the test case contains $n$ integers $a_1, a_2, \dots, a_n$ ($1 \le a_i \le 2 \cdot 10^5$), where $a_i$ is the $i$-th element of $a$.
It is guaranteed that the sum of $n$ does not exceed $2 \cdot 10^5$ ($\sum n \le 2 \cdot 10^5$).
For each test case, print the answer: the length of the shortest prefix of elements you need to erase from $a$ to make it a good array.
Input
The first line of the input contains one integer $t$ ($1 \le t \le 2 \cdot 10^4$) — the number of test cases. Then $t$ test cases follow.
The first line of the test case contains one integer $n$ ($1 \le n \le 2 \cdot 10^5$) — the length of $a$. The second line of the test case contains $n$ integers $a_1, a_2, \dots, a_n$ ($1 \le a_i \le 2 \cdot 10^5$), where $a_i$ is the $i$-th element of $a$.
It is guaranteed that the sum of $n$ does not exceed $2 \cdot 10^5$ ($\sum n \le 2 \cdot 10^5$).
Output
For each test case, print the answer: the length of the shortest prefix of elements you need to erase from $a$ to make it a good array.
Samples
5
4
1 2 3 4
7
4 3 3 8 4 5 2
3
1 1 1
7
1 3 1 4 5 3 2
5
5 4 3 2 3
0
4
0
2
3
Note
In the first test case of the example, the array $a$ is already good, so we don't need to erase any prefix.
In the second test case of the example, the initial array $a$ is not good. Let's erase first $4$ elements of $a$, the result is $[4, 5, 2]$. The resulting array is good. You can prove that if you erase fewer number of first elements, the result will not be good.