input: Write a function to print a staircase of height 'n', where n is an integer. If 'n' is 0, the function should print nothing. If 'n' is negative, the function should print an error message indicating the input is invalid.
  
  def staircase(n): 
      if n < 0:
          print("Invalid input: height cannot be negative.")
      elif n == 0:
          return
      else:
          for i in range(0, n): 
              for j in range(0, n - i - 1): 
                  print(end=" ") 
              for j in range(0, i + 1): 
                  print("#", end="") 
              print("\r") 
    
  assert staircase(5) == None
  assert staircase(0) == None
  assert staircase(-1) == "Invalid input: height cannot be negative."
  assert staircase(3) == None
output: def staircase(n): 
      if n < 0:
          print("Invalid input: height cannot be negative.")
      elif n == 0:
          return
      else:
          for i in range(0, n): 
              for j in range(0, n - i - 1): 
                  print(end=" ") 
              for j in range(0, i + 1): 
                  print("#", end="") 
              print()