leetcode-142. Linked List Cycle II

142. Linked List Cycle II

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

Note: Do not modify the linked list.

Follow up:
Can you solve it without using extra space?

题意：

在142. Linked List Cycle II的基础上，找到环起点的位置。

就是利用287. Find the Duplicate Number的思路，在287题的基础上，判断是否会 不能构成环 即可。具体思路可以去看一下 287题

public class Solution {
    public ListNode detectCycle(ListNode head) {
        if (head == null) return null;
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next.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 fast;
    }
}