From d865e65e7c0d5e817d4bbeb57234aff7faa57f4f Mon Sep 17 00:00:00 2001
From: spencerkrebs <krebsspencer@gmail.com>
Date: Sat, 2 May 2026 14:01:36 -0400
Subject: [PATCH] cycle

---
 linked-list-cycle-ii.py | 32 ++++++++++++++++++++++++++++++++
 nth-node.py             | 18 ++++++++++++++++++
 reverse-linked-list.py  | 33 +++++++++++++++++++++++++++++++++
 3 files changed, 83 insertions(+)
 create mode 100644 linked-list-cycle-ii.py
 create mode 100644 nth-node.py
 create mode 100644 reverse-linked-list.py

diff --git a/linked-list-cycle-ii.py b/linked-list-cycle-ii.py
new file mode 100644
index 00000000..96601f1b
--- /dev/null
+++ b/linked-list-cycle-ii.py
@@ -0,0 +1,32 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution:
+    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head:
+            return None
+        slow = head
+        fast = head
+        found=False
+        while fast and fast.next:
+            fast = fast.next.next
+            slow = slow.next
+            if fast == slow:
+                found=True
+                break
+        # slow and fast will always meet within the first cycle
+        if not found:
+            return None
+        
+        pointer = head
+        while pointer != slow:
+            pointer = pointer.next
+            slow = slow.next
+        
+        return pointer
+        
+
+# O(n) time, O(1) space
\ No newline at end of file
diff --git a/nth-node.py b/nth-node.py
new file mode 100644
index 00000000..ba97926d
--- /dev/null
+++ b/nth-node.py
@@ -0,0 +1,18 @@
+# O(n) time, O(1) space
+class Solution:
+    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+        dummy = ListNode(0,head)
+        right = dummy
+        left = dummy 
+
+        for i in range(n):
+            right = right.next 
+
+
+        while right.next:
+            left = left.next 
+            right = right.next 
+
+        left.next = left.next.next 
+        
+        return dummy.next 
\ No newline at end of file
diff --git a/reverse-linked-list.py b/reverse-linked-list.py
new file mode 100644
index 00000000..90897fe4
--- /dev/null
+++ b/reverse-linked-list.py
@@ -0,0 +1,33 @@
+# O(n) time, O(1) space
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        prev = None
+        cur = head
+        while cur:
+            temp = cur.next 
+            cur.next = prev 
+            prev = cur 
+            cur = temp 
+        return prev 
+
+
+
+# reverse linked list with recursion - O(n) time, O(1) space
+# recursion stack:
+# f(none)
+# f(5)
+# f(4)
+# f(3)
+# f(2)
+# f(1)
+class Solution:
+    def reverseList(self,head):
+        if head is None or head.next is None:
+            return head 
+
+        # result is in global, not a parameter of recursion
+        result = self.reverseList(head.next) # return to returns to the same place from where we called it
+        # 5 5 5 5 5
+        head.next.next = head 
+        head.next = None 
+        return result 
\ No newline at end of file