public class FindFirstIntersectNode {

    public static class Node {
        public int value;
        public Node next;

        public Node(int data) {
            this.value = data;
        }
    }

    public static Node getIntersectNode(Node head1, Node head2) {
        if (head1 == null || head2 == null) {
            return null;
        }
        Node loop1 = getLoopNode(head1);
        Node loop2 = getLoopNode(head2);
        if (loop1 == null && loop2 == null) {
            return noLoop(head1, head2);
        }
        if (loop1 != null & loop2 != null) {
            return bothLoop(head1, loop1, head2, loop2);
        }
        return null;
    }

    public static Node getLoopNode(Node head) {
        if (head == null || head.next == null || head.next.next == null) {
            return null;
        }
        Node slow = head.next;
        Node fast = head.next.next;
        while (fast != slow) {
            if (fast.next == null || fast.next.next == null) {
                return null;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        fast = head;
        while (slow != fast) {
            fast = fast.next;
            slow = slow.next;
        }
        return slow;
    }

    public static Node noLoop(Node head1, Node head2) {
        if (head1 == null || head2 == null) {
            return null;
        }
        Node curr1 = head1;
        Node curr2 = head2;
        int n = 0;
        while (curr1.next != null) {
            n++;
            curr1 = curr1.next;
        }
        while (curr2.next != null) {
            n--;
            curr2 = curr2.next;
        }
        // end node not equal
        if (curr1 != curr2) {
            return null;
        }
        curr1 = n > 0 ? head1 : head2;
        curr2 = curr1 == head1 ? head2 : head1;
        n = Math.abs(n);
        while (n != 0) {
            n--;
            curr1 = curr1.next;
        }
        while (curr1 != curr2) {
            curr1 = curr1.next;
            curr2 = curr2.next;
        }
        return curr1;
    }

    public static Node bothLoop(Node head1, Node loop1, Node head2, Node loop2) {
        Node curr1 = null;
        Node curr2 = null;
        if (loop1 == loop2) {
            curr1 = head1;
            curr2 = head2;
            int n = 0;
            while (curr1 != loop1) {
                n++;
                curr1 = curr1.next;
            }
            while (curr2 != loop2) {
                n--;
                curr2 = curr2.next;
            }
            curr1 = n > 0 ? head1 : head2;
            curr2 = curr1 == head1 ? head2 : head1;
            n = Math.abs(n);
            while (n != 0) {
                n--;
                curr1 = curr1.next;
            }
            while (curr1 != curr2) {
                curr1 = curr1.next;
                curr2 = curr2.next;
            }
            return curr1;
        } else {
            curr1 = loop1.next;
            while (curr1 != loop1) {
                if (curr1 == loop2) {
                    return loop1;
                }
                curr1 = curr1.next;
            }
            return null;
        }
    }

    public static void main(String[] args) {
        // 1->2->3->4->5->6->7->null
		Node head1 = new Node(1);
		head1.next = new Node(2);
		head1.next.next = new Node(3);
		head1.next.next.next = new Node(4);
		head1.next.next.next.next = new Node(5);
		head1.next.next.next.next.next = new Node(6);
		head1.next.next.next.next.next.next = new Node(7);

		// 0->9->8->6->7->null
		Node head2 = new Node(0);
		head2.next = new Node(9);
		head2.next.next = new Node(8);
		head2.next.next.next = head1.next.next.next.next.next; // 8->6
		System.out.println(getIntersectNode(head1, head2).value);

		// 1->2->3->4->5->6->7->4...
		head1 = new Node(1);
		head1.next = new Node(2);
		head1.next.next = new Node(3);
		head1.next.next.next = new Node(4);
		head1.next.next.next.next = new Node(5);
		head1.next.next.next.next.next = new Node(6);
		head1.next.next.next.next.next.next = new Node(7);
		head1.next.next.next.next.next.next = head1.next.next.next; // 7->4

		// 0->9->8->2...
		head2 = new Node(0);
		head2.next = new Node(9);
		head2.next.next = new Node(8);
		head2.next.next.next = head1.next; // 8->2
		System.out.println(getIntersectNode(head1, head2).value);

		// 0->9->8->6->4->5->6..
		head2 = new Node(0);
		head2.next = new Node(9);
		head2.next.next = new Node(8);
		head2.next.next.next = head1.next.next.next.next.next; // 8->6
		System.out.println(getIntersectNode(head1, head2).value);
    }
}