Output the missing integer

[tkellehe]

Output the missing integer

My answer

8 bytes

ɲdFḶṡạĖ⁻

Try it:)

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How it works

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.