C++: Middle of the Linked List

Thought Process

To find the middle of a linked list, we use the two-pointer technique. We initialize two pointers, slow and fast, both starting at the head of the list. The slow pointer moves one step at a time, while the fast pointer moves two steps at a time. When the fast pointer reaches the end of the list, the slow pointer will be at the middle. This approach ensures that we find the middle node in a single pass through the list. 

/**
 * 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* middleNode(ListNode* head) {

        ListNode *slow = head;
        ListNode *fast = head;

        while(fast!=NULL && fast->next!=NULL){

            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }
};

Code Complexity

Time Complexity: O(n)

The algorithm iterates through the linked list once, where 'n' is the number of nodes.

Space Complexity: O(1)

The algorithm uses a constant amount of extra space, regardless of the input size.

Code copied!