#Leetcode# 873. Length of Longest Fibonacci Subsequence

https://leetcode.com/problems/length-of-longest-fibonacci-subsequence/

A sequence X_1, X_2, ..., X_n is fibonacci-like if:

• n >= 3
• X_i + X_{i+1} = X_{i+2} for all i + 2 <= n

Given a strictly increasing array A of positive integers forming a sequence, find the length of the longest fibonacci-like subsequence of A.  If one does not exist, return 0.

(Recall that a subsequence is derived from another sequence A by deleting any number of elements (including none) from A, 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: [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: [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].

Note:

• 3 <= A.length <= 1000
• 1 <= A[0] < A[1] < ... < A[A.length - 1] <= 10^9
• (The time limit has been reduced by 50% for submissions in Java, C, and C++.)

代码：

class Solution {
public:
    int lenLongestFibSubseq(vector<int>& A) {
        unordered_set<int> s(A.begin(), A.end());
        int n = A.size();
        int ans = 0;
        for(int i = 0; i < n; i ++) {
            for(int j = i + 1; j < n; j ++) {
                int st = A[i], en = A[j], len = 2;
                while(s.count(st + en)) {
                    en = st + en;
                    st = en - st;
                    len ++;
                }
                ans = max(ans, len);
            }
        }
        if(ans > 2) return ans;
        else return 0;
    }
};