The key of the day

Problem

"Today's key is hidden in the number," says the message Agent Nieves receives. Her spy network has one fixed rule: the key is the highest digit that appears in the number of the day.

Write a function that takes a whole number and returns its highest digit, as a whole number. With 4192 the digits are 4, 1, 9 and 2: you return 9. The number can be negative, and the sign is not a digit: in -507 the digits are 5, 0 and 7, so you return 7. If the highest digit repeats, you return it once: 5525 gives 5. And 0 has a single digit, 0, so it returns 0.

Examples

• Highest comes in the middle

  4192 → 9

• Highest comes first

  8123 → 8

• Highest comes last

  1237 → 7

• A single digit

  6 → 6

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def highest_digit(n):
    pass

function highestDigit(n) {
}