ɲdFḶṡạĖ⁻
# Input implicitly pushed onto the stack.
ɲd # Pushes the string "0123456789" onto the stack.
FḶṡạĖ⁻ # Loops nine times removing digits from the string "0123456789" producing the missing number.
F # Pushes the string "F" onto the stack.
Ḷ # Consumes the string "F" and converts to a base 98 number producing 9 then loops the following that many times.
ṡ # Swaps the string "0123456789" with the input on the stack.
ạ # Gets the ith element from the input and pushes it onto the top of the stack.
Ė # Grabs the string "0123456789" at the bottom of the stack and puts it at the top.
⁻ # Removes the element pulled out of the input from the string.
# Implicit end of the loop.
# Implicit push to the screen, outputting the missing digit.
Output the missing integer
My answer
8 bytes
Try it:)
How it works