[detect cycle && remove nth && reverse list recursive]

LinkedListCycle.java

@@ -0,0 +1,33 @@
+public class LinkedListCycle {
+    // Time Complexity : O(n)
+    // Space Complexity : O(1)
+    // Did this code successfully run on Leetcode : yes
+    // 2(a + b) = a + b + k(c + b)
+    // a = (k -1 )b + kc
+    //
+    public ListNode detectCycle(ListNode head) {
+        if (head == null || head.next == null ) return null;
+        ListNode slow = head.next;
+        ListNode fast = head.next.next;
+
+        while (fast != slow) {
+            if(fast == null || fast.next == null) {
+                return null;
+            }
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+
+        if(fast == null || fast.next == null) {
+            return null;
+        }
+        slow = head;
+        while (fast != slow) {
+            fast = fast.next;
+            slow = slow.next;
+        }
+        return slow;
+
+
+    }
+}

RemoveNth.java

@@ -0,0 +1,28 @@
+public class RemoveNth {
+    // Time Complexity : O(n)
+    // Space Complexity : O(1)
+    // Did this code successfully run on Leetcode : yes
+    public ListNode removeNthFromEnd(ListNode head, int n) {
+        ListNode dummy = new ListNode(-1);
+        dummy.next = head;
+        ListNode slow  = dummy;
+        ListNode fast  = dummy;
+        if(head == null || head.next == null) {
+            return null;
+        }
+        for(int i = 0; i <= n; i++) {
+            fast = fast.next;
+        }
+
+        while(fast != null) {
+            fast = fast.next;
+            slow = slow.next;
+        }
+        fast = slow.next;
+        slow.next = slow.next.next;
+
+
+        return dummy.next;
+
+    }
+}

ReverseList.java

@@ -0,0 +1,19 @@
+public class ReverseList {
+
+    // Time Complexity : O(n)
+    // Space Complexity : O(h)
+    // Did this code successfully run on Leetcode : yes
+    public ListNode reverseList(ListNode head) {
+        return helper(head);
+    }
+    public ListNode helper(ListNode root) {
+        if (root == null ||  root.next == null) return root;
+
+        ListNode head = helper(root.next);
+        root.next.next = root;
+        root.next = null;
+
+        return head;
+
+    }
+}