input: Write a function to check if an array is sorted.
  assert check_sorted([1, 2, 3, 4, 5]) == True
  assert check_sorted([5, 4, 3, 2, 1]) == False
  assert check_sorted([1, 3, 2, 4, 5]) == False
  assert check_sorted([10, 20, 30, 40, 50]) == True
output: def check_sorted(arr):
      return arr == sorted(arr)