链表中环的入口结点

题目：

如果一个链表中包含环，如何找到环的入口节点？

 

解答：

public class Solution {

    public static ListNode meetingNode(ListNode head) {
        if(head == null) {
            return null;
        }

        ListNode pSlow = head.next;
        if(pSlow == null) {
            return null;
        }

        ListNode pFast = pSlow.next;
        while(pFast != null && pSlow != null) {
            if(pFast == pSlow) {
                return pSLow;
            }

            pSlow = pSlow.next;
            pFast = pFast.next;
            if(pFast.next != null) {
                pFast = pFast.next;
            }
        }

        return null;
    }

    public static ListNode entryNodeOfLoop(ListNode head) {
        ListNode meetingNode = meetingNode(head);
        if(meetingNode == null) {
            return null;
        }

        // 环中的节点的数目
        int nodeInLoop = 1;
        ListNode tmp = meetingNode;
        while(tmp.next != meetingNode) {
            tmp = tmp.next;
            nodeInLoop++;
        }

        // 先移动pNode1
        ListNode pNode1 = head;
        for(int i = 0; i < nodeInLoop; i++) {
            pNode1 = pNode1.next;
        }

        ListNode pNode2 = head;
        while(pNode1 != pNode2) {
            pNode1 = pNode1.next;
            pNode2 = pNode2.next;
        }

        return pNode1;
    }
}