Hi, here's your problem today. This problem was recently asked by Facebook:

You are given an array of integers. Return the smallest positive integer that is not present in the array. The array may contain duplicate entries.

For example, the input [3, 4, -1, 1] should return 2 because it is the smallest positive integer that doesn't exist in the array.

Your solution should run in linear time and use constant space.

Here's your starting point:

<pre lang=py>
def first_missing_positive(nums):
  # Fill this in.

print first_missing_positive([3, 4, -1, 1])
# 2
</pre>