Middle of Linked List(lintcode 228)

Description

Find the middle node of a linked list.

Example

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

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

Interface

/**
 * Definition for ListNode
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
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
    }
}

Solution

/**
 * Definition for ListNode
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
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 head;
        }
        ListNode fast = new ListNode(0);
        fast.next = head;
        ListNode slow = fast;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }

        return slow;
    }
}