Question - Recover corrupted master page

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Question 
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Each page has metadata:

empty bit → whether page is used

offset of next page → pointer (like a linked list)

Example chain:
P1 → P2 → P3 → P10
P2 → P5

Page states:

P1: not empty → next = P3

P2: not empty → next = P5

P3: not empty → next = P10

P4: empty (garbage)

P5: not empty → EOF (sentinel)

P0 = master page
It normally stores starting offsets of all files (i.e., heads of each chain).

But P0 is corrupted, and we must reconstruct the master page.

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Code 
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import java.util.*;

class Page {
    int id;
    boolean isEmpty;
    Integer next; // null means EOF

    Page(int id, boolean isEmpty, Integer next) {
        this.id = id;
        this.isEmpty = isEmpty;
        this.next = next;
    }
}
public class RecoverMasterPage {

    public static List<Integer> recoverMasterPage(List<Page> pages) {

        Set<Integer> used = new HashSet<>();
        Set<Integer> pointed = new HashSet<>();

        // Step 1: collect used pages and next pointers
        for (Page p : pages) {
            if (!p.isEmpty) {
                used.add(p.id);

                if (p.next != null) {
                    pointed.add(p.next);
                }
            }
        }

        // Step 2: start pages = used - pointed
        used.removeAll(pointed);

        // Step 3: return sorted offsets (optional)
        List<Integer> masterOffsets = new ArrayList<>(used);
        Collections.sort(masterOffsets);

        return masterOffsets;
    }

    // Demo
    public static void main(String[] args) {

        List<Page> pages = List.of(
            new Page(1, false, 3),
            new Page(2, false, 5),
            new Page(3, false, 10),
            new Page(4, true, null),   // empty
            new Page(5, false, null)   // EOF
        );

        List<Integer> master = recoverMasterPage(pages);

        System.out.println("Recovered P0 offsets: " + master);
    }
}