adding the problem code

leetcode/p141_linkedlistcycle.cpp

@@ -0,0 +1,26 @@
+/**
+ * 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) {
+        ListNode *p1 = head;
+        ListNode *p2 = head;
+        while(p2!=nullptr && p2->next!=nullptr){
+            // walk one step forward
+            p1 = p1->next;
+            // walk two steps forward
+            p2 = p2->next;
+            p2 = p2->next;
+            if(p1 == p2){
+                return true;
+            }
+        }
+        return false;
+    }
+};