142.环形链表-ii.java

/*
 * @lc app=leetcode.cn id=142 lang=java
 *
 * [142] 环形链表 II
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * class ListNode {
 * int val;
 * ListNode next;
 * ListNode(int x) {
 * val = x;
 * next = null;
 * }
 * }
 */
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 == fast) {
                break;
            }
        }

        // 避免出现slow == fast == head情况
        if (fast == null || fast.next == null) {
            return null;
        } else {
            slow = head;
        }

        while (slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }
}
// @lc code=end