LeetCode

N/A

Posts

Posts

Threads

Likes

Credits

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/

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/