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 }
Leetcode 141 Linked List Cycle
原文：http://www.cnblogs.com/lilixu/p/4589867.html