141-linked-list-cycle

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

/**
 * @param {ListNode} head
 * @return {boolean}
 */

var hasCycle = function(head){

    let slow = head;
    let fast = head;

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

    return false;
}