876.middle-of-the-linked-list.java

/*
 * @lc app=leetcode id=876 lang=java
 *
 * [876] Middle of the Linked List
 *
 * https://leetcode.com/problems/middle-of-the-linked-list/description/
 *
 * algorithms
 * Easy (65.02%)
 * Likes:    618
 * Dislikes: 42
 * Total Accepted:    81.9K
 * Total Submissions: 125.9K
 * Testcase Example:  '[1,2,3,4,5]'
 *
 * 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; }
 * }
 */
class Solution {
  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;
  }
}