SnapshotSet with Iterator

Problem Overview

Design and implement a SnapshotSet<T> data structure that extends the functionality of a standard set by supporting snapshot-based iterators. The key requirement is that iterators capture a snapshot of the set at the time they are created, allowing them to iterate over the elements without being affected by subsequent modifications to the set.

Problem Statement

Implement the SnapshotSet<T> class with the following methods:

1. add(T element): Adds the specified element to the set.
2. remove(T element): Removes the specified element from the set.
3. iterator(): Returns a snapshot-based iterator over the elements in the set.

The iterator should have the following methods:

1. hasNext(): Returns true if the iterator has more elements to traverse.
2. next(): Returns the next element in the iteration.

Examples

`java SnapshotSet<Integer> set = new SnapshotSet<>(); set.add(1); set.add(2); set.add(3);

Iterator<Integer> iterator1 = set.iterator(); // Snapshot of the set at this point while (iterator1.hasNext()) { System.out.println(iterator1.next()); // Prints 1, 2, 3 }

set.add(4); set.remove(2);

Iterator<Integer> iterator2 = set.iterator(); // Snapshot of the set at this point while (iterator2.hasNext()) { System.out.println(iterator2.next()); // Prints 1, 3, 4 } `

Constraints

• The time complexity for add, remove, and iterator should be O(1) on average.
• The space complexity should be minimized.

Hints

• Consider using a combination of a hash map and a linked list to store the elements.
• Use a version number to track the state of the set when the iterator is created.

Solution

`java import java.util.*;

public class SnapshotSet<T> { private static class Node { T value; Node next;

    Node(T value) {
        this.value = value;
    }
}

private Map<T, Node> map;
private Node head;
private int version;

public SnapshotSet() {
    map = new HashMap<>();
    head = new Node(null);
    version = 0;
}

public void add(T element) {
    if (!map.containsKey(element)) {
        Node newNode = new Node(element);
        newNode.next = head.next;
        head.next = newNode;
        map.put(element, newNode);
        version++;
    }
}

public void remove(T element) {
    if (map.containsKey(element)) {
        Node nodeToRemove = map.get(element);
        Node current = head;
        while (current.next != nodeToRemove) {
            current = current.next;
        }
        current.next = nodeToRemove.next;
        map.remove(element);
        version++;
    }
}

public Iterator<T> iterator() {
    return new Iterator<T>() {
        private Node current = head.next;
        private int iteratorVersion = version;

        @Override
        public boolean hasNext() {
            return current != null;
        }

        @Override
        public T next() {
            if (iteratorVersion != version) {
                throw new ConcurrentModificationException();
            }
            T value = current.value;
            current = current.next;
            return value;
        }
    };
}

} `

This solution uses a combination of a hash map and a linked list to store the elements. The hash map allows for efficient lookups, while the linked list maintains the order of insertion. The iterator captures a snapshot of the set by storing the current version number and iterating over the linked list. If the set is modified while iterating, a ConcurrentModificationException is thrown.