[LeetCode][JavaScript]Linked List Cycle II

Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

Follow up:
Can you solve it without using extra space?

https://leetcode.com/problems/linked-list-cycle-ii/

https://leetcode.com/discuss/396/is-there-any-better-answer-for-the-linked-list-cycle-ii

两个指针重合说明有环。

然后把慢指针指向head，双指针每次走一步，再次重合就是结果。

 1 /**
 2  * @param {ListNode} head
 3  * @return {ListNode}
 4  */
 5 var detectCycle = function(head) {
 6     var slow = head, fast = head;
 7     while(true){
 8         if(fast === null || fast.next === null){
 9             return null;
10         }
11         slow = slow.next;
12         fast = fast.next.next;
13         if(slow === fast){
14             slow = head;
15             while(true){
16                 if(slow === fast){
17                     return slow;
18                 }
19                 slow = slow.next;
20                 fast = fast.next;
21             }
22         }
23     }
24 };