2095

2095. Delete the Middle Node of a Linked List (Medium)

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

Similar Questions:

• Remove Nth Node From End of List (Medium)
• Reorder List (Medium)
• Remove Linked List Elements (Easy)
• Middle of the Linked List (Easy)

Solution 1.

// OJ: https://leetcode.com/problems/delete-the-middle-node-of-a-linked-list/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
    int getLength(ListNode *head) {
        int ans = 0;
        for (; head; ++ans, head = head->next);
        return ans;
    }
public:
    ListNode* deleteMiddle(ListNode* head) {
        ListNode dummy, *p = &dummy;
        dummy.next = head;
        int len = getLength(head) / 2;
        for (int i = 0; i < len; ++i, p = p->next);
        p->next = p->next->next;
        return dummy.next;
    }
};

Solution 2. Fast-Slow Pointers

// OJ: https://leetcode.com/problems/delete-the-middle-node-of-a-linked-list/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    ListNode* deleteMiddle(ListNode* head) {
        ListNode dummy, *fast = &dummy, *slow = &dummy;
        dummy.next = head;
        while (fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        slow->next = slow->next->next;
        return dummy.next;
    }
};