142. [链表][双指针]环形链表 II

142. 环形链表II

public class Solution {
    public ListNode detectCycle(ListNode head) {
        if(head == null){
            return null;
        }
        ListNode slow = head, fast = head;
        while(fast != null){
            slow = slow.next;
            if (fast.next == null){
                return null;
            } else {
                fast = fast.next.next;
            }
            if (slow == fast){
                ListNode ptr = head;
                while (ptr != fast){
                    ptr = ptr.next;
                    fast = fast.next;
                }
                return ptr;
            }
        }
        return null;
    }
}