链表 - 快慢指针

var middleNode = function(head) {
    let fast = head,slow = head;
    while(fast !== null && fast.next !== null){

        fast = fast.next.next;
        slow = slow.next
    }
    return slow;
};

var hasCycle = function (head) {
    //快慢指针，模拟发现，必然会在环内相遇，判断相遇就行
    let fast = head, slow = head;

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