# 873.length of longest fibonacci subsequence ## Description

A sequence X_1, X_2, …, X_n is fibonacci-like if:

• n >= 3
• X_i + X_{i+1} = X_{i+2} for all i + 2 <= n

Given a strictly increasing array A of positive integers forming a sequence, find the length of the longest fibonacci-like subsequence of A. If one does not exist, return 0.

(Recall that a subsequence is derived from another sequence A by deleting any number of elements (including none) from A, 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: [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: [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].
``````

### Note:

• 3 <= A.length <= 1000
• 1 <= A < A < … < A[A.length - 1] <= 10^9
• (The time limit has been reduced by 50% for submissions in Java, C, and C++.)

1. updating

## 参考代码

``````class Solution:
def lenLongestFibSubseq(self, A):
length=len(A)
S=0
index=set(A)
for i in range(length-1):
for j in range(i+1,length):
a=A[i]
b=A[j]
s=2
while a+b in index:
ii=a+b
a=b
b=ii
s+=1
S=max(S,s)
return S if S>2 else 0
``````