Recover Sorted Digits from Scrambled English Number Words

A string contains a shuffled collection of letters from the English digit names zero, one, two, three, four, five, six, seven, eight, and nine. Recover the digits and return them in ascending order, preserving repetitions.

Implement recover_digits(letters: string) -> string.

Constraints & Assumptions

• Input has at most 100000 lowercase ASCII letters and is guaranteed to be a valid multiset of complete English digit names. The empty input represents no digits.
• Word boundaries and letter order have been lost. This is not substring segmentation of concatenated intact words.
• Output contains only 0 through 9, sorted nondecreasingly, with one digit per recovered word.
• Aim for O(input length + output length) time and constant-sized counting state, apart from output.

Examples

letters = "owoztneoer"
result = "012"

letters = "fviefuro"
result = "45"

The reported follow-up replaces English digit names with arbitrary customer-provided words for digits 0–9. Explain whether your reasoning still works, what would establish unique recoverability, and how you would detect or report ambiguous mappings. Do not assume arbitrary words contain a distinguishing letter. For example, assigning word a to one digit and word aa to another can make the same letter multiset represent different digit sequences.