142. Linked List Cycle II

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

此题和之前的find the duplicate number比较像，代码如下：

/**
 * 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) {
        ListNode fast = head;
        ListNode slow = head;
        while(fast!=null&&fast.next!=null){
            fast = fast.next.next;
            slow = slow.next;
            if(fast==slow){
                fast = head;
                while(fast!=slow){
                    fast = fast.next;
                    slow = slow.next;
                }
                return fast;
            }
        }
        return null;
    }
}