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876. Middle of the Linked List

Given the head of a singly linked list, return the middle node of the linked list.

If there are two middle nodes, return the second middle node.

 
Example 1:

Input: head = [1,2,3,4,5]
Output: [3,4,5]
Explanation: The middle node of the list is node 3.


Example 2:

Input: head = [1,2,3,4,5,6]
Output: [4,5,6]
Explanation: Since the list has two middle nodes with values 3 and 4, we return the second one.


 
Constraints:


The number of nodes in the list is in the range [1, 100].
1 <= Node.val <= 100
[TOC]
Video Solution


Solution Article

Approach 1: Output to Array
Intuition and Algorithm
Put every node into an array A in order. Then the middle node is just A[A.length // 2], since we can retrieve each node by index.
We can initialize the array to be of length 100, as we're told in the problem description that the input contains between 1 and 100 nodes.

Complexity Analysis

Time Complexity: $$O(N)$$, where $$N$$ is the number of nodes in the given list.
Space Complexity: $$O(N)$$, the space used by A.



Approach 2: Fast and Slow Pointer
Intuition and Algorithm
When traversing the list with a pointer slow, make another pointer fast that traverses twice as fast. When fast reaches the end of the list, slow must be in the middle.

Complexity Analysis

Time Complexity: $$O(N)$$, where $$N$$ is the number of nodes in the given list.
Space Complexity: $$O(1)$$, the space used by slow and fast.





https://leetcode.com/articles/middle-of-...nked-list/