Solution: Count of Subset Sum

class Solution:
  def countSubsets(self, num, s):
    def dp(index, acc, memo):
      
      if (index, acc) in memo:
        return memo[(index, acc)]

      if acc == s:
        return 1

      if index == len(num):
        return 0 

      result = dp(index + 1, acc, memo) + dp(index + 1, acc + num[index], memo)
      memo[(index, acc)] = result

      return result
      
    return dp(0, 0, {})

For "Top-down Dynamic Programming with Memoization", is there a good explanation on using the currentIndex as key first, then sum as key? Why currentIndex should be first?

The alternative would be to use sum as first key and then currentIndex as key.