链表中环的入口结点

/*
 public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {

    public ListNode EntryNodeOfLoop(ListNode pHead) {
        if (pHead == null)
            return null;
        ListNode p1 = pHead, p2 = pHead;
        while (p2 != null && p2.next != null) {
            p1 = p1.next;
            p2 = p2.next.next;
            if (p1 == p2) {
                p2 = pHead;
                while (p1 != p2) {
                    p1 = p1.next;
                    p2 = p2.next;
                }
                if (p1 == p2)
                    return p1;
            }
        }
        return null;
    }
}