Length of Longest Fibonacci Subsequence

A sequenceX_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 astrictly increasing array Aof positive integers forming a sequence, find thelengthof the longest fibonacci-like subsequence ofA. If one does not exist, return 0.

(Recall that a subsequence is derived from another sequenceAby deleting any number of elements (including none) fromA, 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++.)

分析

任意2个数选出来做起点然后一直走下来。最后选最长的

class Solution:
    def lenLongestFibSubseq(self, A: List[int]) -> int:
        res = float('-inf')
        n = len(A)
        ss = set(A)#加快速度
        for i in range(n):
            for j in range(i+1,n):
                a,b,c = A[i],A[j],2
                while a+b in ss:
                    a,b = b,a+b
                    c +=1
                res = max(res,c)
        return res if res > 2 else 0 #没有的情况