876. Middle of the Linked List

Python

Get length and travel to middle

import math


class Solution:
    def middleNode(self, head: Optional[ListNode]) -> Optional[ListNode]:
        length = 0
        cur = head
        while cur:
            length += 1
            cur = cur.next

        target = head
        for i in range(math.ceil(length/2) if length % 2 == 0 else math.floor(length/2)):
            target = target.next

        return target

One Pass Through

(This is be used in 143. Reorder List)

class Solution:
    def middleNode(self, head: Optional[ListNode]) -> Optional[ListNode]:
        left, right = head, head
        while right and right.next:
            left = left.next
            right = right.next.next
        return left

Javascript

var middleNode = function(head) {
  let slow = head;
  let fast = head;
  while (fast !== null && fast.next !==null) {
    slow = slow.next;
    fast = fast.next.next;
  }
  return slow;
};