23.middle-of-the-linked-list.js

// https://leetcode.com/problems/middle-of-the-linked-list/submissions/

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
 var middleNode = function(head) {
    if (head) {
        let slow = head;
        let fast = head;
        
        while(fast && fast.next) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
    
    return head;
};