Middle of Linked List

题目：

Find the middle node of a linked list.

Given 1->2->3, return the node with value 2.
Given 1->2, return the node with value 1.

分析：

无

解法：

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