Today I have done leetcode's February LeetCoding Challenge with cpp.

February LeetCoding Challenge 27

Description

Length of Longest Fibonacci Subsequence

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

class Solution {
public:
  int lenLongestFibSubseq(vector<int>& arr) {
    unordered_map<int, unordered_map<int, int>> mp;
    unordered_set<int> has;
    int len = arr.size();
    int answer = 0;
    for(auto i : arr) has.insert(i);
    for(int i = 2; i < len; ++i) {
      for(int j = i - 1; j >= 0 && arr[j] * 2 > arr[i]; --j) {
        int gt = arr[i];
        int lt = arr[j];
        int diff = gt - lt;
        if(has.count(diff)) {
          mp[gt][lt] = mp[lt][diff] + 1;
          answer = max(mp[gt][lt], answer);
        }
      }
    }
    return answer ? answer + 2 : 0;
  }
};