
# Middle of the Linked List

## Description

[doc](https://leetcode.com/problems/middle-of-the-linked-list/)

## Solutions

### First Idea

```javascript
/**
 * 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 count = 0
  let cur = head

  while (cur !== null) {
    count++
    cur = cur.next
  }

  let middle = Math.floor(count / 2)
  cur = head
  while (middle > 0) {
    cur = cur.next
    middle--
  }

  return cur
}
```

-- Time Complexity: O(n)
-- Space Complexity: O(1)

### Two Pointers

```javascript
/**
 * 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 = (fast = head)

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

    fast = fast.next
    slow = slow.next
  }

  return slow
}
```

-- Time Complexity: O(n)
-- Space Complexity: O(1)
