[LeetCode] 873. Length of Longest Fibonacci Subsequence

Problem

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++.)

Solution

class Solution {
    public int lenLongestFibSubseq(int[] A) {
        Set<Integer> set = new HashSet<>();
        for (int a: A) set.add(a);
        int max = 2;
        for (int i = 0; i < A.length-1; i++) {
            for (int j = i+1; j < A.length; j++) {
                int a1 = A[i], a2 = A[j];
                int curMax = 2;
                while (set.contains(a1+a2)) {
                    curMax++;
                    int temp = a1;
                    a1 = a2;
                    a2 = temp+a2;
                }
                max = Math.max(max, curMax);
            }
        }
        return max == 2 ? 0 : max;
    }
}