Find the Missing Letter

Create a function that takes an array of increasing letters and return the missing letter.

Examples

missingLetter(["a", "b", "c", "e", "f", "g"]) ➞ "d"

missingLetter(["O", "Q", "R", "S"]) ➞ "P"

missingLetter(["t", "u", "v", "w", "x", "z"]) ➞ "y"

missingLetter(["m", "o"]) ➞ "n"

Notes

• Tests will always have exactly one letter missing.
• The length of the test array will always be at least two.
• Tests will be in one particular case (upper or lower but never both).