Find unique common element from 3 arrays

Question

Original Problem:
I have 3 boxes each containing 200 coins, given that there is only one person who has made calls from all of the three boxes and thus the

Answer 1

Use a hashtable mapping objects to frequency counts. Iterate through all three lists, incrementing occurrence counts in the hashtable, until you encounter one with an occurrence count of 3. This is O(n), since no sorting is required. Example in Python:

def find_duplicates(*lists):
  num_lists = len(lists)
  counts = {}
  for l in lists:
    for i in l:
      counts[i] = counts.get(i, 0) + 1
      if counts[i] == num_lists:
        return i

Or an equivalent, using sets:

def find_duplicates(*lists):
  intersection = set(lists[0])
  for l in lists[1:]:
    intersection = intersection.intersect(set(l))
  return intersection.pop()