From 1c5caf7a2ed1758e862e3858c6409c472b43bdff Mon Sep 17 00:00:00 2001
From: nikhylw <1.nikhil.wani+nikhly@gmail.com>
Date: Mon, 4 May 2026 22:49:06 +0530
Subject: [PATCH 1/2] adding reverse_linkedlist

---
 W4_6_reverse_linkedlist_three_pointer.py | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)
 create mode 100644 W4_6_reverse_linkedlist_three_pointer.py

diff --git a/W4_6_reverse_linkedlist_three_pointer.py b/W4_6_reverse_linkedlist_three_pointer.py
new file mode 100644
index 00000000..c6ed501a
--- /dev/null
+++ b/W4_6_reverse_linkedlist_three_pointer.py
@@ -0,0 +1,22 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+
+# Time complexity: O(N), we visit all nodes with curr exactly once. 
+# Space complexity: O(1), we just use a few extra pointers (prev, curr, temp)
+
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        prev = None
+        curr = head
+
+        while curr != None:
+            temp = curr.next # Store curr.next before overiding.
+            curr.next = prev 
+            prev = curr
+            curr = temp # Moving curr to the next node to process
+
+        return prev
+

From 0eaf82c16d62b7a93dea9220a4683c5bd335239c Mon Sep 17 00:00:00 2001
From: nikhylw <1.nikhil.wani+nikhly@gmail.com>
Date: Tue, 5 May 2026 00:17:33 +0530
Subject: [PATCH 2/2] add ll_cycle and ll_delete_nth_last_node_ll

---
 W4_7_cycle_linkedlist.py                | 29 +++++++++++++++++
 W4_8_delete_nth_last_node_linkedlist.py | 43 +++++++++++++++++++++++++
 2 files changed, 72 insertions(+)
 create mode 100644 W4_7_cycle_linkedlist.py
 create mode 100644 W4_8_delete_nth_last_node_linkedlist.py

diff --git a/W4_7_cycle_linkedlist.py b/W4_7_cycle_linkedlist.py
new file mode 100644
index 00000000..98605f9c
--- /dev/null
+++ b/W4_7_cycle_linkedlist.py
@@ -0,0 +1,29 @@
+# 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]:
+        slow = head
+        fast = head
+
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+            if slow == fast:
+                start = head
+
+                while start != slow:
+                    start = start.next
+                    slow = slow.next
+
+                return start
+        
+        return None
+
+# Time complexity O(N)
+# Space complexity O(1)
+
diff --git a/W4_8_delete_nth_last_node_linkedlist.py b/W4_8_delete_nth_last_node_linkedlist.py
new file mode 100644
index 00000000..9eef9414
--- /dev/null
+++ b/W4_8_delete_nth_last_node_linkedlist.py
@@ -0,0 +1,43 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+
+        length = 0 
+        curr = head
+        prev = head
+
+        # Get the length of the linkedlist
+        while curr:
+            curr = curr.next
+            length = length + 1
+        
+        # Get the position of the node to be deleted
+        del_index = (length - n)
+
+        # for cases with difference 0, removing the head.
+        if del_index == 0:
+            return head.next        
+
+        prev = head
+        
+        while del_index > 1:
+            prev = prev.next
+            del_index -= 1
+        
+        curr = prev.next
+
+        # Swaping the links to delete targetnode
+        temp = curr.next
+        curr.next = None
+        prev.next = temp
+
+        return head
+
+# Time complexity: O(2N) = O(N)
+# Space complexity: O(1)
+
+