Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Note: Do not modify the linked list.
Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.
Input: head = [1,2], pos = 0
Output: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Input: head = [1], pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.
Follow-up:
Can you solve it without using extra space?
这是 Floyd’s Tortoise and Hare (Cycle Detection) 算法。
\begin{aligned} 2 \cdot \text { distance }(\text { tortoise }) &=\text { distance }(\text {hare}) \\ 2(F+a) &=F+a+b+a \\ 2 F+2 a &=F+2 a+b \\ F &=b \end{aligned}
这里解释一下：兔子跑的快，相遇时，兔子跑过的距离是乌龟跑过的距离的两倍。兔子沿着环，跑了一圈多，有多跑了 ` ` 才相遇，所以距离是： \(F+a+b+a\)。
这个思路还可以用于解决 287. Find the Duplicate Number。
没想到有这么多环形探测算法！
附加题：尝试其他探测环形算法。
Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Note: Do not modify the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1 Output: tail connects to node index 1 Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2:
Input: head = [1,2], pos = 0 Output: tail connects to node index 0 Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1 Output: no cycle Explanation: There is no cycle in the linked list.
Follow-up:
Can you solve it without using extra space?
link:{sourcedir}/_0142_LinkedListCycleII.java[role=include]