Remove duplicates from array

Choose from the contract

Ask whether the input is sorted, whether original order must remain, and whether the caller expects a new array, count, or in-place prefix. For arbitrary input with stable first occurrences, scan with a set and append only unseen values: O(n) expected time and O(n) space. If sorted input may be compacted in place, keep a write index and copy a value only when it differs from the previous retained value; that is O(n) time and O(1) auxiliary space. Test empty input, one value, all duplicates, duplicates separated in an unsorted list, and values whose equality rule is nontrivial. Do not use the sorted two-pointer algorithm on unsorted data, because equal values may not be adjacent.