链表中环的入口结点

public class Solution {
    public ListNode EntryNodeOfLoop(ListNode pHead) {
        ListNode intersect = getIntersect(pHead);
        if (intersect != null) {
            // 找到相交结点之后只需要与头节点同时前进即可相遇
            while (intersect != pHead) {
                intersect = intersect.next;
                pHead = pHead.next;
            }
        }
        return intersect;
    }
    // 找出快慢指针相交的结点，若没有则返回null
    private ListNode getIntersect(ListNode head) {
        ListNode slow, fast;
        slow = fast = head;
        while (fast != null && fast.next != null) {
            // 快指针一次前进两步，慢指针一次一步
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                return fast;
            }
        }
        return null;
    }