class Solution:
    def longestArithSeqLength(self, A: List[int]) -> int:
        if not A or len(A) == 1:
            return 0
        dic = {}
        for i in range(1, len(A)):
            for j in range(i):
                diff = A[i] - A[j]
                if (diff, j) not in dic:
                    dic[diff, i] = 2
                else:
                    dic[diff, i] = dic[diff, j] + 1
        return max(dic.values())