141. Linked List Cycle

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/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
    if (head === null) return false;
    if (head.next === null) return false;
    
    let slow = head;
    let fast = head.next.next;

    while (fast !== null && fast.next !== null) {
        if (fast === slow) return true;
        slow = slow.next;
        fast = fast.next.next;
    }

    return false;
};