How could the solution be refactored using one collection instead of two (seen and temp)?

Instead of using two collections — seen for already processed elements and temp for temporary storage — you can use a single collection with additional logic to track the state of elements. For example, use a Map with the element as the key and a boolean flag or an enumeration indicating the state (e.g., "processed", "in progress").

Example in Java using HashMap:

Map<String, Boolean> seen = new HashMap<>();

for (String item : items) {
    if (seen.getOrDefault(item, false)) {
        // element already processed, skip
        continue;
    }
    // process the element
    process(item);
    seen.put(item, true);
}

This approach avoids data duplication and simplifies logic by keeping information about processed elements in a single structure.