"""
Note: Write a solution with O(n) time complexity and 
O(1) additional space complexity, since this is what you would 
be asked to do during a real interview.

Given an array a that contains only numbers in the 
range from 1 to a.length, find the first duplicate 
number for which the second occurrence has the minimal 
index. In other words, if there are more than 1 duplicated 
numbers, return the number for which the second 
occurrence has a smaller index than the second 
occurrence of the other number does. If there 
are no such elements, return -1.

"""

def firstDuplicate(numbers): 
    
  pass



nums = [2, 3, 1, 3, 2, 5]

print nums
