/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */


class Solution {
 public:
  ListNode* middleNode(ListNode* head) {
    ListNode* slow = head;
    ListNode* fast = head;

    while (fast && fast->next) {
      slow = slow->next;
      fast = fast->next->next;
    }

    return slow;
  }
};