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+# 873. Length of Longest Fibonacci Subsequence
+from collections import defaultdict
+
+"""
+a sequence 'x1, x2, ..., xn' is fibonacci like if 'n >= 3' and 'x_i + x_i + 1
+== x_i + 2' for all 'i + 2 <= n'. given a strictly increasing '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.
+"""
+
+
+class Solution(object):
+ def lenLongestFibSubseq(self, arr):
+ """
+ :type arr: List[int]
+ :rtype: int
+ """
+ dp, s = defaultdict(int), set(arr)
+ for i in range(len(arr)):
+ for j in range(i):
+ if arr[i] - arr[j] < arr[j] and arr[i] - arr[j] in s:
+ dp[arr[j], arr[i]] = dp.get((arr[i] - arr[j], arr[j]), 2) + 1
+ return max(dp.values() or [0])
+
+
+if __name__ == "__main__":
+ obj = Solution()
+ print(obj.lenLongestFibSubseq(arr=[1, 2, 3, 4, 5, 6, 7, 8]))
+ print(obj.lenLongestFibSubseq(arr=[1, 3, 7, 11, 12, 14, 18]))