1218. Longest Arithmetic Subsequence of Given Difference

https://leetcode.com/problems/longest-arithmetic-subsequence-of-given-difference/

Python

class Solution:
    def longestSubsequence(self, arr: List[int], difference: int) -> int:
        mapper = {}
        ans = 1

        for num in arr:
            cur = 1
            if num - difference in mapper:
                cur += mapper[num-difference]
                ans = max(ans, cur)
            mapper[num] = cur

        return ans