Linked List Cycle II

找到相遇的点之后，只要让两个指针分别从head 和相遇点出发，走同样多的步数再相遇，就找到了链接开始的地方了。

要画个证明一下。

public class Solution {
    public ListNode detectCycle(ListNode head) {
        if(head == null) return null;
        ListNode result = null, slow = head, fast = head;
        int count = 0;
        while(fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            count++;
            if(slow == fast) {
                result = slow;
                break;
            }
        }
        if(result == null) return null;
        slow = head;
        fast = result;
        while(slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }
}