P5679 [GZOI2017] Arithmetic Progression Subsequence

GZOI2017 D2T2

Given a positive integer sequence of length $N$, determine whether there exists an arithmetic progression subsequence with length at least three.

The first line contains a positive integer $T$, indicating the number of test cases. Then follow $T$ test cases. For each test case, the first line contains a positive integer $N$, indicating the length of the sequence. The second line contains $N$ positive integers, in order, representing the numbers in the sequence.

For each test case, output a separate line containing `YES` or `NO`.

Constraints For $20\%$ of the testdata: $N \le 100$. For $40\%$ of the testdata: $N \le 10^3$. For $100\%$ of the testdata: $1 \le T \le 10$, $1 \le N \le 2 \times 10^4$, and each number in the sequence is $\le 2 \times 10^4$. Translated by ChatGPT 5