Middle of Linked List

题目：

Find the middle node of a linked list.

分析：

这题目没什么好说的，给个节点定义：

class ListNode {
  int val;
  ListNode* next;
  ListNode(int x) : val(x), next(NULL) {}
};

解法：

class Solution{
public:
    /**
     * @param head: the head of linked list.
     * @return: a middle node of the linked list
     */
    ListNode *middleNode(ListNode *head) {
        // Write your code here
        if (head == NULL || head->next == NULL) {
            return head;
        }

        ListNode* slow = head;
        ListNode* fast = head;

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

        return slow;
    }
};