Arithmetic Slices II - Subsequence

lcpy gen -n 446   # by problem number
lcpy gen -s arithmetic_slices_ii_subsequence   # by problem name

Input: nums = [2,4,6,8,10]
Output: 7
Explanation: All arithmetic subsequence slices are:
[2,4,6]
[4,6,8]
[6,8,10]
[2,4,6,8]
[4,6,8,10]
[2,4,6,8,10]
[2,6,10]

Input: nums = [7,7,7,7,7]
Output: 16
Explanation: Any subsequence of this array is arithmetic.

class Solution:
    # Time: O(n^2)
    # Space: O(n^2)
    def number_of_arithmetic_slices(self, nums: list[int]) -> int:
        n = len(nums)
        total = 0
        dp: list[dict[int, int]] = [{} for _ in range(n)]
        for i in range(n):
            for j in range(i):
                diff = nums[i] - nums[j]
                count_j = dp[j].get(diff, 0)
                total += count_j
                dp[i][diff] = dp[i].get(diff, 0) + count_j + 1
        return total