141. Linked List Cycle

Given a linked list, determine if it has a cycle in it.

这个就可以使用快慢指针了，快指针一次走两步，慢指针一次走一步，如果快指针直接就到最后一个节点了，那肯定是没有环，如果什么时候快指针又和慢指针重合了，那就是有环。

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