快慢指针

为什么相遇后让快指针回到起点，再让快指针和慢指针都一步一步地走，它们就会在环的入口相遇？

复杂度

• 时间复杂度:  O(n)

• 空间复杂度: O(1)

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