From 4c845dc5e9c72a3faa74fd91330d1cd46f9f3dcc Mon Sep 17 00:00:00 2001
From: varnika-98 <varnikaprasad98@gmail.com>
Date: Mon, 12 Jan 2026 16:31:18 -0800
Subject: [PATCH 1/4] Completed Exercise 1: Stack implementation

---
 Exercise_1.py | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/Exercise_1.py b/Exercise_1.py
index 532833f5d..905d2d894 100644
--- a/Exercise_1.py
+++ b/Exercise_1.py
@@ -1,21 +1,29 @@
 class myStack:
-  #Please read sample.java file before starting.
-  #Kindly include Time and Space complexity at top of each file
+  #Time Complexity: push: O(1), pop: O(1), peek: O(1), isEmpty: O(1), size: O(1), show: O(n)
+  #Space Complexity: O(n)
+
      def __init__(self):
+         self.items = []
          
      def isEmpty(self):
+         return len(self.items) == 0
          
      def push(self, item):
+         self.items.append(item)
          
      def pop(self):
-        
-        
+         if not self.isEmpty():
+             return self.items.pop()
+         
      def peek(self):
-        
+         if not self.isEmpty():
+             return self.items[-1]
+         
      def size(self):
+         return len(self.items)
          
      def show(self):
-         
+         print(self.items)
 
 s = myStack()
 s.push('1')

From ea6e0eeed792f161f882fd36750ebd5b554ed066 Mon Sep 17 00:00:00 2001
From: varnika-98 <varnikaprasad98@gmail.com>
Date: Mon, 12 Jan 2026 16:33:01 -0800
Subject: [PATCH 2/4] Add .gitignore to exclude macOS files

---
 .gitignore | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 .gitignore

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 000000000..e43b0f988
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+.DS_Store

From 2858012fb3881e40561a8cc5061529ed841d32e8 Mon Sep 17 00:00:00 2001
From: varnika-98 <varnikaprasad98@gmail.com>
Date: Mon, 12 Jan 2026 16:50:36 -0800
Subject: [PATCH 3/4] Completed Exercise 2: Linked list stack

---
 Exercise_2.py | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/Exercise_2.py b/Exercise_2.py
index b11492215..c5b16a634 100644
--- a/Exercise_2.py
+++ b/Exercise_2.py
@@ -1,15 +1,28 @@
 
 class Node:
+#TIME: push: O(1), pop: O(1), __init__ : O(1)
+#SPACE: O(n)
     def __init__(self, data):
        self.data = data
        self.next = None
  
 class Stack:
     def __init__(self):
+        self.top = None
         
     def push(self, data):
+        new_node = Node(data)
+        new_node.next = self.top
+        self.top = new_node
         
     def pop(self):
+        if self.isEmpty():
+            return None
+        popped_node = self.top
+        self.top = self.top.next
+        return popped_node.data
+        
+    
         
 a_stack = Stack()
 while True:

From 61dff69000280dc3e73141ca82b5e9514eafaa86 Mon Sep 17 00:00:00 2001
From: varnika-98 <varnikaprasad98@gmail.com>
Date: Mon, 12 Jan 2026 17:05:15 -0800
Subject: [PATCH 4/4] Completed Exercise 3: Singly linked list

---
 Exercise_3.py | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/Exercise_3.py b/Exercise_3.py
index a5d466b59..1d0969699 100644
--- a/Exercise_3.py
+++ b/Exercise_3.py
@@ -1,8 +1,12 @@
 class ListNode:
+    #TIME: append: O(n), find: O(n), remove: O(n)
+    #SPACE: O(1)
     """
     A node in a singly-linked list.
     """
     def __init__(self, data=None, next=None):
+        self.data = data
+        self.next = next
     
 class SinglyLinkedList:
     def __init__(self):
@@ -13,12 +17,29 @@ def __init__(self):
         self.head = None
 
     def append(self, data):
+        new_node = ListNode(data)
+        # If list is empty
+        if self.head is None:
+            self.head = new_node
+            return
+        # Traverse to the end of the list
+        current = self.head
+        while current.next:
+            current = current.next
+        current.next = new_node
+
         """
         Insert a new element at the end of the list.
         Takes O(n) time.
         """
         
     def find(self, key):
+        current = self.head
+        while current:
+            if current.data == key:
+                return current
+            current = current.next
+        return None
         """
         Search for the first element with `data` matching
         `key`. Return the element or `None` if not found.
@@ -26,7 +47,33 @@ def find(self, key):
         """
         
     def remove(self, key):
+        current = self.head
+        previous = None
+
+        #if head contains the key
+        if current and current.data == key:
+            self.head = current.next
+            return
+        #search for the key to be deleted
+        while current and current.data != key:
+            previous = current
+            current = current.next
+        #if key was not present in linked list
+        if current is None:
+            return
+        #unlink the node from linked list
+        previous.next = current.next
         """
         Remove the first occurrence of `key` in the list.
         Takes O(n) time.
         """
+
+
+
+ll = SinglyLinkedList() 
+ll.append(10) 
+ll.append(20) 
+ll.append(30) 
+print(ll.find(20).data) # 20 
+ll.remove(20) 
+print(ll.find(20)) # None
\ No newline at end of file