Leetcode 141 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?

LL经典题：一个指针每次前进1node，另一个指针每次前进2node，如果有环则必会到达同一node。

var hasCycle = function(head) {
    if(!head) 
        return false
    var a = head
    var b = head
    while(b.next && b.next.next){
        b = b.next.next
        a = a.next
        if(a===b)
            return true
    }
    return false
}