复习-JZ55 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 || pHead.next == null)
            return null;

        ListNode p1 = pHead;
        ListNode p2 = pHead;

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

}