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# 0873. Length of Longest Fibonacci Subsequence

<https://leetcode.com/problems/length-of-longest-fibonacci-subsequence>

## Description

A sequence `x1, x2, ..., xn` is *Fibonacci-like* if:

* `n >= 3`
* `xi + xi+1 == xi+2` for all `i + 2 <= n`

Given a **strictly increasing** array `arr` of positive integers forming a sequence, return *the **length** of the longest Fibonacci-like subsequence of* `arr`. If one does not exist, return `0`.

A **subsequence** is derived from another sequence `arr` by deleting any number of elements (including none) from `arr`, without changing the order of the remaining elements. For example, `[3, 5, 8]` is a subsequence of `[3, 4, 5, 6, 7, 8]`.

**Example 1:**

```
**Input:** arr = [1,2,3,4,5,6,7,8]
**Output:** 5
**Explanation:** The longest subsequence that is fibonacci-like: [1,2,3,5,8].
```

**Example 2:**

```
**Input:** arr = [1,3,7,11,12,14,18]
**Output:** 3
**Explanation**:The longest subsequence that is fibonacci-like: [1,11,12], [3,11,14] or [7,11,18].
```

**Constraints:**

* `3 <= arr.length <= 1000`
* `1 <= arr[i] < arr[i + 1] <= 109`

## ac

```java
```