# 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]))