【剑指Offer】23. 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 fast = pHead, slow = pHead;
        do{
            fast = fast.next.next;
            slow = slow.next;
        }while(fast != slow);
        
        // 此时x=z（fast从头节点开始，slow从第一次相遇z点开始。下一次相遇则是入口）
        fast = pHead;
        while(fast != slow) {
            fast = fast.next;
            slow = slow.next;
        }
       
        return slow;
    }
}