LL141LinkedListCycle

思路

1. use two pointers, one is fast, one is slow.
   
   • When the two meet, it has cycle
   • Bug:
     
     • Have to make sure the pointer is not null, and its next is not null before assigning the pointer, e.g. fast = fast.null
     • hasCycleTwoPointers2() optimize the while condition.

Remember

• if ListNode ln = new ListNode(3), then ln.next == null. The next pointer is always exists. So we don’t need to check ln.next != null if we assign fast = ln.next;

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

• Remove element while iterating the set is not elegant. http://stackoverflow.com/questions/1110404/remove-elements-from-a-hashset-while-iterating