0876-middle-of-the-linked-list.c

/**
 * 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.
 *
 * Constraints:
 *
 * The number of nodes in the list is in the range [1, 100].
 * 1 <= Node.val <= 100
 *
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 *
 * Space = O(1)
 * Time = O(n)
 */

struct ListNode* middleNode(struct ListNode* head){
    struct ListNode* slow = head;
    struct ListNode* fast = head;

    while (fast && fast->next) {
        slow = slow->next;
        fast = fast->next->next;
    }

    return slow;
}