Linked List Cycle ||

循环链表||

题目

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?

解析重点

1.为了知道是否有循环，我们使用通用的方式使用快慢指针。
2.这里重要的是快慢指针相交的点c，他到head点的距离和循环的距离是相等的，那么从head点和交点c出发，他们相等的点就是交点。

java代码

/**
 * 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) {
	ListNode slow = head, fast = head;
	while(fast != null && fast.next != null) {
		fast = fast.next.next;
		slow = slow.next;
		if (slow == fast) {
			while (head != slow) {
				head = head.next;
				slow = slow.next;
			}
			return slow;				
		}
	}			
	return null;
}
}