剑指 Offer II 022. 链表中环的入口节点

其实本题的难点在于如果数学推导。

public class Solution {
    public ListNode detectCycle(ListNode head) {
        // 入口节点
        if (head == null) return null;
        ListNode fast = head, slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            // 环中相遇
            if (fast == slow) {
                fast = head;
                while (fast != slow) {
                    fast = fast.next;
                    slow = slow.next;
                }
                return fast;
            }
        }
        return null;
    }
}