JZ55 链表中环的入口结点

经典做法，原理：加法交换律。

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

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

    public ListNode EntryNodeOfLoop(ListNode pHead){
        ListNode slow=pHead, fast=pHead;
        while(fast!=null&&fast.next!=null){
            slow=slow.next;
            fast=fast.next.next;
            if(slow==fast) break;
        }
        if(fast==null||fast.next==null) return null;
        slow=pHead;
        while(fast!=slow){
            fast=fast.next;
            slow=slow.next;
        }
        return fast;
    }
}