sum of absolute differences of a number in an array

Question

I want to calculate the sum of absolute differences of a number at index i with all integers up to index i-1 in o(n). But i am not able to think of any approach better than o(n^

Answer 1

I can offer an O(n log n) solution for a start: Let f_i be the i-th number of the result. We have:

When walking through the array from left to right and maintain a binary search tree of the elements a₀ to a_i-1, we can solve all parts of the formula in O(log n):

• Keep subtree sizes to count the elements larger than/smaller than a given one
• Keep cumulative subtree sums to answer the sum queries for elements larger than/smaller than a given one

We can replace the augmented search tree with some simpler data structures if we want to avoid the implementation cost:

• Sort the array beforehand. Assign every number its rank in the sorted order
• Keep a binary indexed tree of 0/1 values to calculate the number of elements smaller than a given value
• Keep another binary indexed tree of the array values to calculate the sums of elements smaller than a given value

TBH I don't think this can be solved in O(n) in the general case. At the very least you would need to sort the numbers at some point. But maybe the numbers are bounded or you have some other restriction, so you might be able to implement the sum and count operations in O(1).

An implementation:

# binary-indexed tree, allows point updates and prefix sum queries
class Fenwick:
  def __init__(self, n):
    self.tree = [0]*(n+1)
    self.n = n
  def update_point(self, i, val):  # O(log n)
    i += 1
    while i <= self.n:
      self.tree[i] += val
      i += i & -i
  def read_prefix(self, i):        # O(log n)
    i += 1
    sum = 0
    while i > 0:
      sum += self.tree[i]
      i -= i & -i
    return sum

def solve(a):
  rank = { v : i for i, v in enumerate(sorted(a)) }
  res = []
  counts, sums = Fenwick(len(a)), Fenwick(len(a))
  total_sum = 0
  for i, x in enumerate(a):
    r = rank[x]
    num_smaller = counts.read_prefix(r)
    sum_smaller = sums.read_prefix(r)
    res.append(total_sum - 2*sum_smaller + x * (2*num_smaller - i))
    counts.update_point(r, 1)
    sums.update_point(r, x)
    total_sum += x
  return res

print(solve([3,5,6,7,1]))  # [0, 2, 4, 7, 17]
print(solve([2,0,1]))      # [0, 2, 2]