题解：CF633D Fibonacci-ish

原题

题目描述

Yash has recently learnt about the Fibonacci sequence and is very excited about it. He calls a sequence Fibonacci-ish if

1. the sequence consists of at least two elements
2. $ f_{0} $ and $ f_{1} $ are arbitrary
3. $ f_{n+2}=f_{n+1}+f_{n} $ for all $ n>=0 $ .

You are given some sequence of integers $ a_{1},a_{2},…,a_{n} $ . Your task is rearrange elements of this sequence in such a way that its longest possible prefix is Fibonacci-ish sequence.

输入格式

The first line of the input contains a single integer $ n $ ( $ 2<=n<=1000 $ ) — the length of the sequence $ a_{i} $ .

The second line contains $ n $ integers $ a_{1},a_{2},…,a_{n} $ ( $ |a_{i}|<=10^{9} $ ).

输出格式

Print the length of the longest possible Fibonacci-ish prefix of the given sequence after rearrangement.

输入输出样例 #1

输入 #1

3
1 2 -1

输出 #1

3

输入输出样例 #2

输入 #2

5
28 35 7 14 21

输出 #2

4

说明/提示

In the first sample, if we rearrange elements of the sequence as $ -1 $ , $ 2 $ , $ 1 $ , the whole sequence $ a_{i} $ would be Fibonacci-ish.

In the second sample, the optimal way to rearrange elements is , , , , $ 28 $ .

思路

先看数据范围 $2\le n\le 1000$，直接暴力加剪枝就能过。

我们首先用 map 建一个桶，统计每个数出现的次数，然后用双重 for 循环枚举前两个数。

然后，我们再来看如何剪枝：

• 如果 2