LinkedListCycleII.java

package easy;

import util.ListNode;
/**
 * 
 * ClassName: LinkedListCycleII
 * @author chenyiAlone  
 * Create Time: 2019/03/04 20:28:42
 * Description: No.142
 * Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
    To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
    Note: Do not modify the linked list.
    
    Example 1:
    
    Input: head = [3,2,0,-4], pos = 1
    Output: tail connects to node index 1
    Explanation: There is a cycle in the linked list, where tail connects to the second node.
    
    Example 2:
    
    Input: head = [1,2], pos = 0
    Output: tail connects to node index 0
    Explanation: There is a cycle in the linked list, where tail connects to the first node.
    
    Example 3:
    
    Input: head = [1], pos = -1
    Output: no cycle
    Explanation: There is no cycle in the linked list.
 */
public class LinkedListCycleII {
    public ListNode detectCycle(ListNode head) {
        ListNode fast = head, slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                fast = head;
                while (fast != slow) {
                    fast = fast.next;
                    slow = slow.next;
                }
                return fast;
            }
        }
        return null;
    }
}