环形链表

[pwstrick]

141. 环形链表

/**
 * 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;
    let fast = head.next,
        slow = head;
    while(fast != slow) {
        if(fast == null || fast.next == null)
            return false;
        slow = slow.next;
        fast = fast.next.next;
    }
    return true;
};