链表中环的入口节点（Java实现）

public class E23FindEntryFromLoop {
    //链表中环的入口节点

    private class ListNode{
        int value;
        ListNode nextNode;
    }

    public static ListNode entryNodeOfLoop(ListNode head){
        if (head == null)
            return null;
        //判断是否存在环
        ListNode loopNode = meetingNode(head);
        if (loopNode == null)
            return null;
        //得到环中节点数量
        int count = 1;
        ListNode node = loopNode;
        while(node.nextNode != loopNode){
            loopNode = loopNode.nextNode;
            count ++;
        }
        //找出环的入口节点
        ListNode ahead = head;
        ListNode behind = head;
        for (int i = 0; i < count; i ++)
            ahead = ahead.nextNode;
        while(ahead != behind){
            ahead = ahead.nextNode;
            behind = behind.nextNode;
        }
        return behind;
    }

    private static ListNode meetingNode(ListNode head){
        //判断是否存在环并返回一个处于环中的节点，不存在则返回null
        ListNode fast = head;
        ListNode slow = head;

        fast = fast.nextNode;
        while(slow != null && fast != null){
            if (slow == fast)
                return slow;
            slow = slow.nextNode;
            fast = fast.nextNode;
            if (fast != null)
                fast = fast.nextNode;
        }
        return null;
    }
}