Question

Java: Given two sets, LinkedSet this and LinkedSet that, consisting of a generic type of linked lists, write the method that returns the union of "this" and "that". You can use the LinkedSet constructor to construct a new LinkedSet that represents the union of the two.

public Set<E> union(Set<E> that) { }

private LinkedSet(LinkedNode head) {
    this.head = head;
}

Answer 1

public class LinkedSet<T> implements Set<T> {
        /*
         * inner class to represent node
         */
        private class LinkedNode {
                private T data;
                private LinkedNode next;

                public LinkedNode(T item) {
                        data = item;
                        next = null;
                }
        }

        private LinkedNode head;
        private int size;

        /*
         * adds item to bag if not already present
         * returns true if item successfully added, false if not
         */
        public boolean add(T item) {
                // to keep the last element
                LinkedNode prev = null;
                LinkedNode start = head;
                while (start != null) {
                        if (start.data.equals(item)) {
                                return false;
                        }
                        prev = start;
                        start = start.next;
                }

                if (head == null) {
                        head = new LinkedNode(item);
                } else {
                        prev.next = new LinkedNode(item);
                }
                size++;
                return true;
        }

        /*
         * removes item from set
         * returns removed item
         */
        public T remove() {
                if (size == 0) {
                        return null;
                }

                T removed = head.data;
                head = head.next;
                size--;
                return removed;
        }

        /*
         * returns item from set
         */
        public T get() {
                if (size == 0) {
                        return null;
                }

                return head.data;
        }

        /*
         * returns true if item is in set, false otherwise
         */
        public boolean contains(T item) {
                LinkedNode current = head;

                for (int i = 0; i < size; i++) {
                        if (current.data.equals(item)) {
                                return true;
                        }

                        current = current.next;
                }

                return false;
        }
        
        public Set<T> union(Set<T> that) { 
                LinkedSet<T> result = new LinkedSet<>();

                LinkedNode l1 = head;
                while(l1 != null) {
                        result.add(l1.data);
                        l1 = l1.next;
                }
                LinkedNode l2 = head;
                while(l2 != null) {
                        if(!result.contains(l2.data))
                                result.add(l2.data);
                        l2 = l2.next;
                }
                
                return result;
        }

        /*
         * returns size of set
         */
        public int size() {
                return size;
        }

        /*
         * returns string representation of contents of set
         */
        public String toString() {
                if (size == 0) {
                        return "[ ]";
                }

                String s = "[";

                LinkedNode current = head;

                while (current.next != null) {
                        s += current.data + ", ";

                        current = current.next;
                }

                return s + current.data + "]";
        }

        /*
         * checks if item is null and throws exception if so
         */
        private void checkForNull(T item) {
                if (item == null) {
                        throw new IllegalArgumentException("null not a possible value!");
                }
        }
}

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