# Time:  O(n^2)
# Space: O(n^2)

import collections

class Solution(object):
    def longestArithSeqLength(self, A):
        """
        :type A: List[int]
        :rtype: int
        """
        dp = collections.defaultdict(int)
        for i in xrange(len(A)-1):
            for j in xrange(i+1, len(A)):
                v =  A[j]-A[i]
                dp[v, j] = max(dp[v, j], dp[v, i]+1)
        return max(dp.values())+1