Linked_list_cycle_ii.java

package com.netease.kaola.act.compose;

public class Linked_list_cycle_ii {

    public static class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
            next = null;
        }

        ListNode(int x, ListNode next) {
            this.val = x;
            this.next = next;
        }
    }

    public ListNode detectCycle(ListNode head) {
        if (head == null || head.next == null) {
            return null;
        }
        if (head.next == head) {
            return head;
        }
        ListNode slow = head;
        ListNode fast = head;
        while (slow != null && fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                break;
            }
        }
        if (slow == null || fast == null) {
            return null;
        }
        if (slow == head) {
            return head;
        }
        for (slow = head; slow != null && fast != null; ) {
            slow = slow.next;
            fast = fast.next;
            if (slow == fast) {
                return slow;
            }
        }
        return null;
    }
}