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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;}