code: def combination_sum_topdown(nums, target):
      """Find number of possible combinations in nums that add up to target, in top-down manner.
  
      Keyword arguments:
      nums -- positive integer array without duplicates
      target -- integer describing what a valid combination should add to
      """
      global DP
      DP = [-1] * (target + 1)
      DP[0] = 1
      return helper_topdown(nums, target)
  
label_desc: Use image node_id 2 for calling a global function with example usage: combination_sum_topdown(nums, target) and returns: helper_topdown