Note: Do not modify the linked list.
Follow up:
Can you solve it without using extra space?
判断一个链表中是否有环存在,并且输出环的起始点。我们用快慢指针fast,slow来判断是否有环存在,如果它们可以相遇就说明有环存在,如果不能相遇说明没有环存在。它们相遇之后,图中的meet点是它们的相遇点,假设join为环的起始点,我们假设head到join的距离为a, join到meet的距离为b(逆时针),meet到join的距离为c(逆时针)。如果它们相遇,那么fast走过的路程是slow走过的两倍,所以a + b + c + b = 2 ( a + b),即a = c。有了这个关系我们只需要用两个指针一个从相遇点出发,另一个从头结点出发,它们相遇的点就是环的起始点。代码如下:
/** * Definition for singly-linked list. * class ListNode { * int val; * ListNode next; * ListNode(int x) { * val = x; * next = null; * } * } */ public class Solution { public ListNode detectCycle(ListNode head) { if(head == null || head.next == null) return null; ListNode fast = head; ListNode slow = head; while(fast != null && fast.next != null) { slow = slow.next; fast = fast.next.next; if(slow == fast) break; } if(fast != slow) return null; fast = head; while(fast != slow) { fast = fast.next; slow = slow.next; } return fast; } }