Middle of the Linked List - Practice Coding Problems

Middle of the Linked List - Problem

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.

Input & Output

Example 1 — Odd Length List

$ Input: head = [1,2,3,4,5]

› Output: [3,4,5]

💡 Note: The middle node is 3. Since there's only one middle node in odd-length lists, return node 3 and all nodes after it.

Example 2 — Even Length List

$ Input: head = [1,2,3,4,5,6]

› Output: [4,5,6]

💡 Note: There are two middle nodes (3 and 4). Return the second middle node (4) and all nodes after it.

Example 3 — Single Node

$ Input: head = [1]

› Output: [1]

💡 Note: Only one node exists, so it is the middle node.

Constraints

The number of nodes in the list is in the range [1, 100]
1 ≤ Node.val ≤ 100

Visualization

Tap to expand

Understanding the Visualization

Input

Linked list [1,2,3,4,5]

Find Middle

Use fast/slow pointers to locate middle

Output

Return middle node and all following nodes

Key Takeaway

🎯 Key Insight: Fast and slow pointers eliminate the need to count nodes first

Asked in

G Google 25 a Amazon 20 M Microsoft 15 🍎 Apple 12

The key insight is using the fast and slow pointer technique (tortoise and hare). While the two-pass approach works, the optimal solution uses two pointers moving at different speeds to find the middle in a single pass. Time: O(n), Space: O(1)

Common Approaches

Approach	Time	Space	Notes
✓ Two Pointers - Fast and Slow	O(n)	O(1)	Use fast and slow pointers to find middle in one pass
Two Pass - Count Then Find	O(n)	O(1)	Count total nodes, then traverse to the middle position

Two Pointers - Fast and Slow — Algorithm Steps

Step 1: Initialize fast and slow pointers at head
Step 2: Move fast 2 steps, slow 1 step each iteration
Step 3: When fast reaches end, slow is at middle

Visualization

Tap to expand

Step-by-Step Walkthrough

Initialize

Both pointers start at head

Move

Fast moves 2 steps, slow moves 1 step

Middle Found

When fast reaches end, slow is at middle

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct ListNode {
    int val;
    struct ListNode* next;
};

struct ListNode* solution(struct ListNode* head) {
    if (!head) return NULL;
    
    struct ListNode* slow = head;
    struct ListNode* fast = head;
    
    while (fast && fast->next) {
        slow = slow->next;
        fast = fast->next->next;
    }
    
    return slow;
}

int main() {
    char line[10000];
    fgets(line, sizeof(line), stdin);
    
    // Parse array
    int arr[1000];
    int size = 0;
    char* token = strtok(line, "[],\n");
    while (token) {
        arr[size++] = atoi(token);
        token = strtok(NULL, "[],\n");
    }
    
    // Build linked list
    struct ListNode* head = NULL;
    if (size > 0) {
        head = (struct ListNode*)malloc(sizeof(struct ListNode));
        head->val = arr[0];
        head->next = NULL;
        struct ListNode* current = head;
        for (int i = 1; i < size; i++) {
            current->next = (struct ListNode*)malloc(sizeof(struct ListNode));
            current = current->next;
            current->val = arr[i];
            current->next = NULL;
        }
    }
    
    // Find middle
    struct ListNode* result = solution(head);
    
    // Find position and print result
    if (result) {
        struct ListNode* current = head;
        int pos = 0;
        while (current != result) {
            current = current->next;
            pos++;
        }
        printf("[");
        for (int i = pos; i < size; i++) {
            if (i > pos) printf(",");
            printf("%d", arr[i]);
        }
        printf("]\n");
    } else {
        printf("[]\n");
    }
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single pass through the list, fast pointer visits each node once

✓ Linear Growth

Space Complexity

O(1)

Only uses two pointer variables

✓ Linear Space

425.0K Views

High Frequency

~10 min Avg. Time

8.5K Likes

Ln 1, Col 1

Smart Actions

💡 Explanation

AI Ready

💡 Suggestion Tab to accept Esc to dismiss

// Output will appear here after running code

Code Editor Closed

Click the red button to reopen

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Two Pointers - Fast and Slow — Algorithm Steps

Visualization

Code -

Time & Space Complexity

Select Compiler