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https://leetcode.com/problems/length-of-longest-fibonacci-subsequence/
class Solution(object):
def lenLongestFibSubseq(self, A):
"""
:type A: List[int]
:rtype: int
"""
s = set(A)
cnt = 0
for i in range(len(A)):
for j in range(i+1, len(A)):
a,b = A[i],A[j]
t = 2
while a+b in s:
a, b = b, a+b
t = t+1
if t>=3:
cnt = max(cnt, t)
return cnt