873. Length of Longest Fibonacci Subsequence

Medium

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] <= 10^9

解題

func lenLongestFibSubseq(arr []int) int {
    m := make(map[int]bool)
    ans := 0

    for _, n := range arr {
        m[n] = true
    }   

    for i:=0; i<len(arr); i++ {
        for j:=i + 1; j<len(arr); j++ {
            first, second, third := arr[i], arr[j], arr[i] + arr[j]
            length := 2
            
            for m[third] {
                first = second
                second = third
                third = first + second
                length++
            }
            ans = max(ans, length)
        }
    }

    if ans < 3 { return 0 }
    return ans
}

func max(a, b int) int {
    if a > b {
        return a
    }
    return b
}