MiddleOfTheLinkedList876.java

/**
 * Given a non-empty, singly linked list with head node head, return a middle
 * node of linked list.
 * 
 * If there are two middle nodes, return the second middle node.
 * 
 * Example 1:
 * 
 * Input: [1,2,3,4,5]
 * Output: Node 3 from this list (Serialization: [3,4,5])
 * The returned node has value 3.  (The judge's serialization of this node is [3,4,5]).
 * Note that we returned a ListNode object ans, such that:
 * ans.val = 3, ans.next.val = 4, ans.next.next.val = 5, and ans.next.next.next = NULL.
 * 
 * Example 2:
 * 
 * Input: [1,2,3,4,5,6]
 * Output: Node 4 from this list (Serialization: [4,5,6])
 * Since the list has two middle nodes with values 3 and 4, we return the second one.
 * 
 * Note:
 * The number of nodes in the given list will be between 1 and 100.
 */

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */

public class MiddleOfTheLinkedList876 {
    public ListNode middleNode(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        
        return slow;
    }

}