PermMissingElem Find the missing element in a given permutation. Input / Output [] -> 1 [1, 2] -> 3 [2, 1, 3] -> 4 [2, 3, 1, 5] -> 4 [2, 3, 4, 5, 6] -> 1 [4, 2, 3, 5, 1, 6, 8, 9] -> 7