141. Linked List Cycle

LinkedListCycle/step1.md

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+```java
+// Create a hashSet to store all found node, if we find the same node again meaning there is a cycle then return true.
+// Time complexity: O(N) N: number of node
+// Space complexity: O(N)
+// Time spent: 04:50
+public class Solution {
+    public boolean hasCycle(ListNode head) {
+        Set<ListNode> foundNode = new HashSet<>();
+
+        while (head != null) {
+            if (foundNode.contains(head)) {
+                return true;        
+            } else {
+                foundNode.add(head);
+                head = head.next;
+            }
+        }
+
+        return false;
+    }
+}
+```

LinkedListCycle/step2.md

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+```java
+public class Solution {
+    public boolean hasCycle(ListNode head) {
+        Set<ListNode> visitedNode = new HashSet<>();
+
+        while (head != null) {
+            if (visitedNode.contains(head)) {
+                return true;        
+            }
+            visitedNode.add(head);
+            head = head.next;
+        }
+
+        return false;
+    }
+}
+```
+
+```java
+// After checking solutions, found a faster O(1) space solution using two pointers
+// Create 2 pointers, 1 pointer is trying to visit next node, and other pointer is trying to visit next next node
+// If there is a cycle, the two-step pointer will eventually catch up with the slow pointer
+//        Node1 -> Node2 -> Node3 -> Node4 -> Node5 -> Node2
+// Index   0        1        2        3        4        5
+//                  5        6        7        8        9
+// Index of one-step pointer: 0(n1), 1(n2), 2(n3), 3(n4), 4(n5) 
+// Index of two-step pointer: 0(n1), 2(n3), 4(n5), 6(n3), 8(n5)
+// Time complexity: O(N) N: number of node
+// Space complexity: O(1)
+public class Solution {
+    public boolean hasCycle(ListNode head) {
+        ListNode oneStepNode = head;
+        ListNode twoStepNode = head;
+
+        while ((oneStepNode != null) && (twoStepNode != null) && (twoStepNode.next != null)) {
+            oneStepNode = oneStepNode.next;
+            twoStepNode = twoStepNode.next.next;
+
+            if (oneStepNode == twoStepNode) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}
+```
+
+```java
+// only need to check nullable condition for twoStepNode since it runs faster
+public class Solution {
+    public boolean hasCycle(ListNode head) {
+        ListNode oneStepNode = head;
+        ListNode twoStepNode = head;
+
+        while ((twoStepNode != null) && (twoStepNode.next != null)) {
+            oneStepNode = oneStepNode.next;
+            twoStepNode = twoStepNode.next.next;
+
+            if (oneStepNode == twoStepNode) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}
+```
+

LinkedListCycle/step3.md

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+```java
+// 1st try: time spend: 01:25
+// 2nd try: time spend: 01;28
+// 3rd try: time spend: 01:12
+```
+