/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode* middleNode(ListNode* head) {        ListNode *slow = head, *fast = head;        while (fast != NULL && fast->next != NULL) {            slow = slow->next;            fast = fast->next->next;        }        return slow;    }};