[LeetCode][JavaScript]Linked List Cycle

Linked List Cycle 

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

Follow up:
Can you solve it without using extra space?

https://leetcode.com/problems/linked-list-cycle/

---

判断链表是否有环。

先是用了额外空间的做法。

移出每个访问过的节点，指向头节点，如果遇到了指向头节点的点，说明有环。

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
    var h = new ListNode("head");
    if(!head){
        return false;
    }
    if(head.next && head.next == head){
        return true;
    }
    return cycle(head);

    function cycle(node){
        if(node === null || node.next === null){
            return false;
        }else if(node.next.val === h.val){
            return true;
        }else{
            var tmp = node.next;
            node.next = h;
            return cycle(tmp);
        }
    }
};

不用额外空间：

https://leetcode.com/discuss/1566/by-saying-using-no-extra-space-does-it-mean-o-0-in-space

使用快慢指针，如果重合则说明有环。

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