# 873. Length of Longest Fibonacci Subsequence

Medium
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] <= 10^9`

### 解題

func lenLongestFibSubseq(arr []int) int {
m := make(map[int]bool)
ans := 0
for _, n := range arr {
m[n] = true
}
for i:=0; i<len(arr); i++ {
for j:=i + 1; j<len(arr); j++ {
first, second, third := arr[i], arr[j], arr[i] + arr[j]
length := 2
for m[third] {
first = second
second = third
third = first + second
length++
}
ans = max(ans, length)
}
}
if ans < 3 { return 0 }
return ans
}
func max(a, b int) int {
if a > b {
return a
}
return b
}