package Leetcode_LinkedList;

/**
 * 題目：
 *      Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
 *      Note: Do not modify the linked list.
 *      Follow up:Can you solve it without using extra space?
 */
public class DeleteCycle_93_1014 {
    public ListNode DeleteCycle(ListNode head) {
        if (head == null) {
            return head;
        }


        ListNode slow = head;
        ListNode fast = head;

        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;

            if (slow == fast) {
                fast = head;
                while (slow != fast) {
                    slow = slow.next;
                    fast = fast.next;
                }
                return slow;
            }
        }
        return null;
    }

    public static void main(String[] args) {
        ListNode node1 = new ListNode(1);
        ListNode node2 = new ListNode(2);
        ListNode node3 = new ListNode(3);
        ListNode node4 = new ListNode(4);
        ListNode node5 = new ListNode(5);
        ListNode node6 = new ListNode(6);
        ListNode node7 = new ListNode(7);

        node1.next = node2;
        node2.next = node3;
        node3.next = node4;
        node4.next = node5;
        node5.next = node6;
        node6.next = node7;
        node7.next = node4;

        DeleteCycle_93_1014 test = new DeleteCycle_93_1014();
        ListNode result = test.DeleteCycle(node1);
        System.out.println(result.val);
    }
}