From e5439714fddc8fe126952aa3ce5ddcf40e6b75b9 Mon Sep 17 00:00:00 2001
From: allurkarsneha <allurkars25@gmail.com>
Date: Mon, 27 Jul 2026 19:14:38 -0500
Subject: [PATCH] Complete PreCourse-1 exercises in Python

---
 Exercise_1.py | 24 +++++++++++++-
 Exercise_2.py | 17 +++++++++-
 Exercise_3.py | 87 +++++++++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 124 insertions(+), 4 deletions(-)

diff --git a/Exercise_1.py b/Exercise_1.py
index 532833f5d..4975bc093 100644
--- a/Exercise_1.py
+++ b/Exercise_1.py
@@ -1,20 +1,42 @@
+# Time Complexity:
+# isEmpty: O(1)
+# push: O(1) amortized
+# pop: O(1)
+# peek: O(1)
+# size: O(1)
+# show: O(n)
+
+# Space Complexity: O(n)
+
 class myStack:
   #Please read sample.java file before starting.
   #Kindly include Time and Space complexity at top of each file
      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()
+          else:
+              return None
         
      def peek(self):
+          if not self.isEmpty():
+              return self.items[-1]
+          else:
+              return None
         
      def size(self):
+          return len(self.items)
          
      def show(self):
+          return self.items
          
 
 s = myStack()
diff --git a/Exercise_2.py b/Exercise_2.py
index b11492215..53d1b29f9 100644
--- a/Exercise_2.py
+++ b/Exercise_2.py
@@ -1,3 +1,8 @@
+# Time Complexity:
+# push: O(1)
+# pop: O(1)
+
+# Space Complexity: O(n)
 
 class Node:
     def __init__(self, data):
@@ -6,11 +11,21 @@ def __init__(self, data):
  
 class Stack:
     def __init__(self):
+         self.head = None
         
     def push(self, data):
+        new_node = Node(data)
+        new_node.next = self.head
+        self.head = new_node
         
     def pop(self):
-        
+        if self.head is None:
+            return None
+        else:
+            popped = self.head.data
+            self.head = self.head.next
+            return popped
+
 a_stack = Stack()
 while True:
     #Give input as string if getting an EOF error. Give input like "push 10" or "pop"
diff --git a/Exercise_3.py b/Exercise_3.py
index a5d466b59..0264150c1 100644
--- a/Exercise_3.py
+++ b/Exercise_3.py
@@ -1,8 +1,19 @@
+# Time Complexity:
+# append: O(n)
+# find: O(n)
+# remove: O(n)
+
+# Space Complexity:
+# O(n) for storing n nodes
+# O(1) extra space for each operation
+
 class ListNode:
     """
     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):
@@ -17,16 +28,88 @@ def append(self, data):
         Insert a new element at the end of the list.
         Takes O(n) time.
         """
-        
+        new_node = ListNode(data)
+        if not self.head:
+            self.head = new_node
+        else:
+            current = self.head
+            while current.next:
+                current = current.next
+            current.next = new_node
+
     def find(self, key):
         """
         Search for the first element with `data` matching
         `key`. Return the element or `None` if not found.
         Takes O(n) time.
         """
-        
+        current = self.head
+        while current:
+            if current.data == key:
+                return current
+            current = current.next
+        return None
+
     def remove(self, key):
         """
         Remove the first occurrence of `key` in the list.
         Takes O(n) time.
         """
+        current = self.head
+        previous = None
+        while current:
+            if current.data == key:
+                if previous:
+                    previous.next = current.next
+                else:
+                    self.head = current.next
+                current.next = None
+                return current
+            previous = current
+            current = current.next
+        return None
+
+if __name__ == "__main__":
+    linked_list = SinglyLinkedList()
+
+    print("Empty list tests:")
+    print(linked_list.find(10))     
+    print(linked_list.remove(10))   
+    linked_list.append(10)
+    linked_list.append(20)
+    linked_list.append(30)
+    linked_list.append(40)
+
+    print("\nFind tests:")
+
+    found = linked_list.find(20)
+    print(found.data if found else None)  
+
+    found = linked_list.find(40)
+    print(found.data if found else None)   
+
+    found = linked_list.find(99)
+    print(found.data if found else None)  
+
+    linked_list.remove(10)
+    print("\nAfter removing head 10:")
+    print(linked_list.head.data)            
+
+    linked_list.remove(30)
+    print("\nAfter removing middle value 30:")
+    print(linked_list.find(30))            
+
+    linked_list.remove(40)
+    print("\nAfter removing tail 40:")
+    print(linked_list.find(40))         
+
+    print("\nRemaining head:")
+    print(linked_list.head.data)      
+
+    linked_list.remove(99)
+    print("\nAfter trying to remove 99:")
+    print(linked_list.head.data)           
+
+    linked_list.remove(20)
+    print("\nAfter removing final node:")
+    print(linked_list.head)                 
\ No newline at end of file