LeetCode/Solutions/Linked List/142. Linked List Cycle II/Solution.java at master · worldlightyjx/LeetCode · GitHub

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/**
 * 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) {
        if (head == null || head.next == null)
            return null;
        ListNode fast = head;
        ListNode slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                slow = head;
                while (fast != slow) {
                    slow = slow.next;
                    fast = fast.next;
                }
                return slow;
            }
        }
        return null;

    }
}