input: Write a Python function that helps determine the first absent positive number.
  assert find_first_missing_positive([1,2,3,-1,5], 5) == 4
  assert find_first_missing_positive([0,-1,-2,1,5,8], 6) == 2
  assert find_first_missing_positive([0,1,2,5,-8], 5) == 3
output: def find_first_missing_positive(arr, n):
      for i in range(n):
          while 1 <= arr[i] <= n and arr[arr[i] - 1] != arr[i]:
              arr[arr[i] - 1], arr[i] = arr[i], arr[arr[i] - 1]
      for i in range(n):
          if arr[i] != i + 1:
              return i + 1
      return n + 1