Given an array A of integers, return the length of the longest arithmetic subsequence in A.
Recall that a subsequence of A is a list A[i_1], A[i_2], ..., A[i_k] with 0 <= i_1 < i_2 < ... < i_k <= A.length - 1, and that a sequence B is arithmetic if B[i+1] - B[i] are all the same value (for 0 <= i < B.length - 1).
 
Example 1:
Input: A = [3,6,9,12]
Output: 4
Explanation: 
The whole array is an arithmetic sequence with steps of length = 3.

Example 2:
Input: A = [9,4,7,2,10]
Output: 3
Explanation: 
The longest arithmetic subsequence is [4,7,10].

Example 3:
Input: A = [20,1,15,3,10,5,8]
Output: 4
Explanation: 
The longest arithmetic subsequence is [20,15,10,5].

 
Constraints:

2 <= A.length <= 1000
0 <= A[i] <= 500

```python

USER_FILENAME = "<user_code>"
import sys, json, builtins
import math, heapq, bisect, itertools, functools, collections, statistics, fractions, decimal, operator, array, random, re, string, os, pathlib
try:
    import resource as _res
except Exception:
    _res = None
from collections import Counter, defaultdict, deque, OrderedDict
from functools import lru_cache, reduce
from itertools import accumulate, permutations, combinations, product
from math import gcd, ceil, floor, sqrt
from decimal import Decimal
try:
    from typing import *
except Exception:
    pass
try:
    sys.setrecursionlimit(1_000_000)
except Exception:
    pass
try:
    import site as _site
    try:
        _sys_sites = _site.getsitepackages() if hasattr(_site, "getsitepackages") else []
        for _p in _sys_sites:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
    try:
        _usp = _site.getusersitepackages()
        if isinstance(_usp, str):
            _usp = [_usp]
        for _p in _usp:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
except Exception:
    pass

def _disabled_input(*args, **kwargs):
    raise RuntimeError("input() is disabled in callable mode")
builtins.input = _disabled_input

ns = {"__name__": "__judge__"}  # prevent if __name__ == "__main__" blocks
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = "class Solution:\n    def longestArithSeqLength(self, A: List[int]) -> int:\n        \n        if len(A)==1:\n            return 1\n        if len(A)==2:\n            return 2\n        N=len(A)\n        ans=2\n        seen={}\n        for i in range(N):\n            for j in range(i+1,N):\n                diff=A[j]-A[i]\n                # now for the rest can we do this diff?\n                \n                if (i,diff) in seen:\n                    ans=max(ans,seen[i,diff]+1)\n                    seen[j,diff]=seen[i,diff]+1\n                else:\n                    seen[j,diff]=2\n                \n                \n                # # prev=A[j]\n                # # for k in range(j+1,N):\n                # #     if A[k]-prev==diff:\n                # #         curr+=1\n                # #         prev=A[k]\n                # ans=max(ans,curr)\n                \n            # seen.add(A[i])\n        return ans\n                    \n        \n#         if N<=2:\n#             return N\n        \n#         dp=[0]*N\n        \n#         dp[2]=3 if A[2]-A[1]==A[1]-A[0] else 0\n        \n#         ans=dp[2]\n        \n#         for i in range(3,N):\n#             if A[i]-A[i-1]==A[i-1]-A[i-2]:\n#                 ans=max(ans,dp[i-1]+1,3)\n#                 dp[i]=max(1+dp[i-1],3)\n        \n#         return ans\n        \n#         N=len(A)\n#         def rec(diff,i,m):\n#             if (i,diff) in m:\n#                 return m[i,diff]\n            \n#             if i==N:\n#                 return 0\n            \n#             m[i,diff]=0\n            \n#             ans=0\n            \n#             for k in range(i+1,N):\n#                 if A[k]-A[i]==diff:\n#                     ans=1+rec(diff,k+1,m)\n#                     break\n            \n#             # make a new diff\n#             # ans=max(ans,rec(diff,i+1,m))\n            \n#             for k in range(i+1,N):\n#                 ans=max(ans,2+rec(A[k]-A[i],k+1,m))\n            \n#             m[i,diff]=ans\n#             return ans\n        \n#         return rec(float('inf'),0,{})\n            \n#             if A[i]-diff in seen and seen[A[i]-diff]==i-:\n#                 seen[A[i]]=i\n#                 print('found',A[i],diff,i,seen)\n#                 m[i,diff]=1+rec(diff,i+1,seen,m)\n#             else:\n#                 m[i,diff]=rec(diff,i+1,seen,m)\n            \n#             return m[i,diff]\n                \n            \n            \n        \n        \n#         N=len(A)\n#         ans=0\n#         m={}\n#         for i in range(N):\n#             for j in range(i+1,N):\n#                 diff=A[j]-A[i]\n#                 if diff!=0:\n#                     seen={A[j]:1, A[i]:0}\n#                     ans=max(ans,2+rec(diff,j+1,seen,m))\n        \n#         return ans\n"
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'longestArithSeqLength'
if _fn_name:
    if "Solution" in ns:
        try:
            _obj = ns["Solution"]()
            if hasattr(_obj, _fn_name):
                _target = getattr(_obj, _fn_name)
        except Exception:
            _target = None
    if _target is None and _fn_name in ns and callable(ns[_fn_name]):
        _target = ns[_fn_name]
# Fallback: if there's exactly one user-defined function, use it
if _target is None:
    _cands = [v for k, v in ns.items() if callable(v) and getattr(v, "__module__", "") in ("__judge__", "__main__")]
    _user_funcs = [c for c in _cands if getattr(c, "__name__", "").startswith("_") is False]
    if len(_user_funcs) == 1:
        _target = _user_funcs[0]
if _target is None:
    raise RuntimeError(f"Could not resolve callable '{_fn_name}'")

_raw = sys.stdin.read().strip()
if _raw:
    _parsed = json.loads(_raw)
    if isinstance(_parsed, dict):
        _kwargs = _parsed.get("kwargs", {})
        _args = _parsed.get("args", [])
    else:
        _args, _kwargs = _parsed, {}
else:
    _args, _kwargs = [], {}
if not isinstance(_args, (list, tuple)):
    _args = [_args]
_res = _target(*_args, **_kwargs)
try:
    sys.stdout.write(repr(_res) + "\n")
except Exception:
    pass

```

Measured memory_bytes: 5964