876.Middle-of-the-Linked-List

876. Middle of the Linked List

题目地址

https://leetcode.com/problems/middle-of-the-linked-list/

题目描述

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.

代码

Approach #1 Output to Array

Time O(N) Space O(N)

/**
 * 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[] A = new ListNode[100];
    int t = 0;
    while (head != null) {
      A[t++] = head;
      head = head.next;
    }

    return A[t / 2];
  }
}

Approach #2 Fast and Slow Pointer

Time O(N) Space O(1)

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