CTCI 2.4

/* Use two additional sentinel pointer to store the node less than x and greater or equal to x.
Then linked the two parition list together. The time complexity is O(N) and the space complexity is
O(1).
*/

public class ParitionLinkedList {
    public Node parition(Node head, int x) {
        Node head1 = new Node(0);
        Node head2 = new Node(0);
        Node temp1 = head1, temp2 = head2, temp = head;
        while(head != null) {
            temp = head.next;
            head.next = null;
            if(head.val < x) {
                temp1.next = head;
                temp1 = temp1.next;
            }
            else {
                temp2.next = head;
                temp2 = temp2.next;
            }
            head = temp;
        }
        //Be careful if there is no element less than x, making head1.next = null
        if(temp1 != head1) {
            temp1.next = head2.next;
            return head1.next;
        }
        else return head2.next;
    }

    public void print(Node head) {
        Node temp = head;
        while(temp != null) {
            System.out.print(temp.val + " ");
            temp = temp.next;
        }
        System.out.println("");
    }

    public static void main(String[] args) {
        Node node1 = new Node(1); Node node11 = new Node(1); Node node111 = new Node(1);
        Node node2 = new Node(2); Node node22 = new Node(2); Node node3 = new Node(3);
        node1.next = node11; node11.next = node2; node2.next = node111; node111.next = node3; node3.next = node22;
        Node head = node1;
        ParitionLinkedList pl = new ParitionLinkedList();
        pl.print(pl.parition(head, 4));
    }
}