142.java

/**
 * Definition for singly-linked list.
 * class ListNode {
 * int val;
 * ListNode next;
 * ListNode(int x) {
 * val = x;
 * next = null;
 * }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;

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

        return null;

    }
}

// 第二次
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        while(fast!=null&&fast.next!=null){
            slow = slow.next;
            fast = fast.next.next;
            if(fast==slow){
                while(head!=slow){
                    head = head.next;
                    slow = slow.next;
                }
                return slow;
            }
        }
        return null;
        
    }
}

// 第三次
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        ListNode slow = head;
        ListNode fast = slow;
        while(slow!=null&&fast!=null&&fast.next!=null){
            slow = slow.next;
            fast = fast.next.next;
            if(slow==fast){
              while(head!=slow){
                slow = slow.next;
                head = head.next;
              }
              return slow;
            }
        }
        return null;

    }
}