Add 'Linked List Cycle'.

Author: AnnieKim

LinkedListCycle.h

@@ -0,0 +1,36 @@
+/*
+ Author:     Annie Kim, [email protected]
+ Date:       Nov 9, 2013
+ Problem:    Linked List Cycle
+ Difficulty: Easy
+ Source:     http://oj.leetcode.com/problems/linked-list-cycle/
+ Notes:
+ Given a linked list, determine if it has a cycle in it.
+ Follow up:
+ Can you solve it without using extra space?
+
+ Solution: two pointers.
+*/
+
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    bool hasCycle(ListNode *head) {
+        if (!head || !head->next) return false;
+        ListNode *back = head, *front = head->next->next;
+        while (true) 
+        {
+            if (front == back) return true;
+            if (!front || !front->next) return false;
+            front = front->next->next;
+            back = back->next;
+        }
+    }
+};