output_description: Print the number of possible combinations of colors of the N people's hats,
  modulo 1000000007.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      A_1 A_2 A_3 ... A_N
attempt: import sys
  
  bbn = 1000000007
  
  # +++++
  
  
  def main():
      n = int(input())
      aal = list(map(int, input().split()))
      cc = [0, 0, 0]
      total_count = 1
      for nh in aal:
          nc = cc.count(nh)
          total_count = (total_count * nc) % bbn
          if total_count == 0:
              return 0
  
          ii = cc.index(nh)
          # pa((cc, total_count,ii,nh,nc))
          cc[ii] += 1
  
      print(total_count)
  
  
  # +++++
  isTest = False
  
  
  def pa(v):
      if isTest:
          print(v)
  
  
  if __name__ == "__main__":
      if sys.platform == "ios":
          sys.stdin = open("inputFile.txt")
          isTest = True
      else:
          pass
          # input = sys.stdin.readline
  
      ret = main()
      if ret is not None:
          print(ret)
  
problem_description: Statement
  
  N people are standing in a queue, numbered 1, 2, 3, ..., N from front to back.
  Each person wears a hat, which is red, blue, or green.
  
  The person numbered i says:
  
    * "In front of me, exactly A_i people are wearing hats with the same color as mine."
  
  Assuming that all these statements are correct, find the number of possible
  combinations of colors of the N people's hats.
  
  Since the count can be enormous, compute it modulo 1000000007.
samples: [{"input": "6\n    0 1 2 3 4 5", "output": "3\n    \n\nWe have three possible combinations, as follows:\n\n  * Red, Red, Red, Red, Red, Red\n  * Blue, Blue, Blue, Blue, Blue, Blue\n  * Green, Green, Green, Green, Green, Green\n\n* * *"}, {"input": "3\n    0 0 0", "output": "6\n    \n\n* * *"}, {"input": "54\n    0 0 1 0 1 2 1 2 3 2 3 3 4 4 5 4 6 5 7 8 5 6 6 7 7 8 8 9 9 10 10 11 9 12 10 13 14 11 11 12 12 13 13 14 14 15 15 15 16 16 16 17 17 17", "output": "115295190"}]