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873. Length of Longest Fibonacci Subsequence

Difficulty Topics

Description

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] <= 109

Solution

length-of-longest-fibonacci-subsequence.cpp
class Solution {
public:
   int lenLongestFibSubseq(vector<int>& A) {
        unordered_set<int> s(A.begin(), A.end());
        int res = 0;
        for (int i = 0; i < A.size(); i++){
            for (int j = i+1; j < A.size(); j++){
                int first = A[i], second = A[j], l = 2;

                while (s.count(first+second)){
                    second = first+second, first = second-first, ++l;
                    res = max(res, l);
                }
            }
        }

       return res > 2 ? res : 0;
    }
};