# Posts published in “Fast and Slow Pointers Traversal”

Given a non-empty, singly linked list with head node `head`, return a middle node of linked list.

If there are two middle nodes, return the second middle node.

Example 1:

```Input: [1,2,3,4,5]
Output: Node 3 from this list (Serialization: [3,4,5])
The returned node has value 3.  (The judge's serialization of this node is [3,4,5]).
Note that we returned a ListNode object ans, such that:
ans.val = 3, ans.next.val = 4, ans.next.next.val = 5, and ans.next.next.next = NULL.
```

Example 2:

```Input: [1,2,3,4,5,6]
Output: Node 4 from this list (Serialization: [4,5,6])
Since the list has two middle nodes with values 3 and 4, we return the second one.
```

Note:

• The number of nodes in the given list will be between `1` and `100`.

Idea:

Use fast and slow pointers traversal

Solution:

``````/**
* Definition for singly-linked list.
* function ListNode(val, next) {
*     this.val = (val===undefined ? 0 : val)
*     this.next = (next===undefined ? null : next)
* }
*/
/**
* @param {ListNode} head
* @return {ListNode}
*/
var middleNode = function(head) {
let slow = head;
let fast = head;

while(fast) {
if (!fast.next) break;
fast = fast.next.next;
slow = slow.next;
}

return slow;
};``````

Given `head`, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the `next` pointer. Internally, `pos` is used to denote the index of the node that tail’s `next` pointer is connected to. Note that `pos` is not passed as a parameter.

Return `true` if there is a cycle in the linked list. Otherwise, return `false`.

Example 1:

```Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).
```

Example 2:

```Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.
```

Example 3:

```Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.
```

Constraints:

• The number of the nodes in the list is in the range `[0, 104]`.
• `-105 <= Node.val <= 105`
• `pos` is `-1` or a valid index in the linked-list.

Can you solve it using `O(1)` (i.e. constant) memory?

Idea:

Use fast and slow pointers traversal, you they can meet each other then there is a circle

Solution:

``````/**
* Definition for singly-linked list.
* function ListNode(val) {
*     this.val = val;
*     this.next = null;
* }
*/

/**
* @param {ListNode} head
* @return {boolean}
*/
var hasCycle = function(head) {
let fast = head;
let slow = head;

while (fast) {
if (!fast.next) return false;
fast = fast.next.next;
slow = slow.next;
if (fast === slow) return true;
}
return false;
};``````