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Weekly Contest 270 Q2
Linked List
Two Pointers

2095. Delete the Middle Node of a Linked List

中文文档

Description

You are given the head of a linked list. Delete the middle node, and return the head of the modified linked list.

The middle node of a linked list of size n is the ⌊n / 2⌋th node from the start using 0-based indexing, where ⌊x⌋ denotes the largest integer less than or equal to x.

  • For n = 1, 2, 3, 4, and 5, the middle nodes are 0, 1, 1, 2, and 2, respectively.

 

Example 1:

Input: head = [1,3,4,7,1,2,6]
Output: [1,3,4,1,2,6]
Explanation:
The above figure represents the given linked list. The indices of the nodes are written below.
Since n = 7, node 3 with value 7 is the middle node, which is marked in red.
We return the new list after removing this node.

Example 2:

Input: head = [1,2,3,4]
Output: [1,2,4]
Explanation:
The above figure represents the given linked list.
For n = 4, node 2 with value 3 is the middle node, which is marked in red.

Example 3:

Input: head = [2,1]
Output: [2]
Explanation:
The above figure represents the given linked list.
For n = 2, node 1 with value 1 is the middle node, which is marked in red.
Node 0 with value 2 is the only node remaining after removing node 1.

 

Constraints:

  • The number of nodes in the list is in the range [1, 105].
  • 1 <= Node.val <= 105

Solutions

Solution 1: Fast and Slow Pointers

The fast and slow pointer technique is a common method used to solve problems related to linked lists. We can maintain two pointers, a slow pointer $\textit{slow}$ and a fast pointer $\textit{fast}$. Initially, $\textit{slow}$ points to a dummy node, whose $\textit{next}$ pointer points to the head node $\textit{head}$ of the list, while $\textit{fast}$ points to the head node $\textit{head}$.

Then, we move the slow pointer one position backward and the fast pointer two positions backward each time, until the fast pointer reaches the end of the list. At this point, the node next to the node pointed by the slow pointer is the middle node of the list. We can remove the middle node by setting the $\textit{next}$ pointer of the node pointed by the slow pointer to point to the next next node.

The time complexity is $O(n)$, where $n$ is the length of the list. The space complexity is $O(1)$.

Python3

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def deleteMiddle(self, head: Optional[ListNode]) -> Optional[ListNode]:
        dummy = ListNode(next=head)
        slow, fast = dummy, head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        slow.next = slow.next.next
        return dummy.next

Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode deleteMiddle(ListNode head) {
        ListNode dummy = new ListNode(0, head);
        ListNode slow = dummy, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        slow.next = slow.next.next;
        return dummy.next;
    }
}

C++

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* deleteMiddle(ListNode* head) {
        ListNode* dummy = new ListNode(0, head);
        ListNode* slow = dummy;
        ListNode* fast = head;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        slow->next = slow->next->next;
        return dummy->next;
    }
};

Go

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func deleteMiddle(head *ListNode) *ListNode {
	dummy := &ListNode{Val: 0, Next: head}
	slow, fast := dummy, dummy.Next
	for fast != nil && fast.Next != nil {
		slow, fast = slow.Next, fast.Next.Next
	}
	slow.Next = slow.Next.Next
	return dummy.Next
}

TypeScript

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

function deleteMiddle(head: ListNode | null): ListNode | null {
    const dummy = new ListNode(0, head);
    let [slow, fast] = [dummy, head];
    while (fast && fast.next) {
        slow = slow.next;
        fast = fast.next.next;
    }
    slow.next = slow.next.next;
    return dummy.next;
}