Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

《程式設計師面試金典》原題

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public
 
 class Solution {
    public ListNode detectCycle(ListNode head) {
        if(null==head) return null;
        ListNode fast=head;
        ListNode slow=head;
        while(fast!=null&&fast.next!=null){
            fast=fast.next.next;
            slow=slow.next;
            if(fast==slow){
                break
 
;
            }
        }
        if(fast==null||fast.next==null){
            return null;
        }
        fast=head;
        while(fast!=slow){
            fast=fast.next;
            slow=slow.next;
        }
        return slow;
    }
}