diff --git a/Exercise_1.py b/Exercise_1.py
index 532833f5d..d012937e1 100644
--- a/Exercise_1.py
+++ b/Exercise_1.py
@@ -1,24 +1,60 @@
+"""
+Time Complexity:
+- push: O(1)
+- pop: O(1)
+- peek: O(1)
+- size: O(1)
+
+Space Complexity:
+- O(n) where n = capacity
+- Here capacity is fixed at 10000, so space complexity is O(1) in practical terms.
+
+Simple fixed-capacity stack implementation.
+"""
+
 class myStack:
-  #Please read sample.java file before starting.
-  #Kindly include Time and Space complexity at top of each file
-     def __init__(self):
-         
-     def isEmpty(self):
-         
-     def push(self, item):
-         
-     def pop(self):
-        
-        
-     def peek(self):
-        
-     def size(self):
-         
-     def show(self):
-         
-
-s = myStack()
-s.push('1')
-s.push('2')
-print(s.pop())
-print(s.show())
+    # Please read sample.java file before starting.
+    # Kindly include Time and Space complexity at top of each file
+    def __init__(self, capacity=10000):
+        self.capacity = capacity
+        self.stack = [None] * capacity
+        self.top = -1
+
+    def isFull(self):
+        return self.top == self.capacity - 1
+
+    def isEmpty(self):
+        return self.top == -1
+
+    def push(self, item):
+        if self.isFull():
+            raise OverflowError("Stack is full. Stack Overflow")
+        self.top += 1
+        self.stack[self.top] = item
+
+    def pop(self):
+        if self.isEmpty():
+            raise IndexError("Stack is empty. Stack Underflow")
+        item = self.stack[self.top]
+        self.stack[self.top] = None
+        self.top -= 1
+        return item
+
+    def peek(self):
+        if self.isEmpty():
+            raise IndexError("Stack is empty.")
+        return self.stack[self.top]
+
+    def size(self):
+        return self.top + 1
+
+    def show(self):
+        return [self.stack[i] for i in range(self.top + 1)]
+
+
+if __name__ == "__main__":
+    s = myStack()
+    s.push('1')
+    s.push('2')
+    print(s.pop())
+    print(s.show())
diff --git a/Exercise_2.py b/Exercise_2.py
index b11492215..41ede011a 100644
--- a/Exercise_2.py
+++ b/Exercise_2.py
@@ -1,4 +1,18 @@
+"""
+1. Push
+    Time complexity: O(1) - constant number of pointer updates
+    regardless of stack size.
+    
+    Space complexity: O(1) - one new node is allocated per call;
+    does not depend on the current size of the stack.
 
+2. Pop
+    Time complexity: O(1) - constant number of pointer updates
+    regardless of stack size.
+    
+    Space complexity: O(1) - one new node is allocated per call;
+    does not depend on the current size of the stack.
+"""
 class Node:
     def __init__(self, data):
        self.data = data
@@ -6,11 +20,35 @@ def __init__(self, data):
  
 class Stack:
     def __init__(self):
+        self.head = None
+        self.size = 0
         
-    def push(self, data):
         
+    def push(self, data):
+        """
+        Approach: Create a new node, point its `next` at the
+        current head (the old top of the stack), then make the
+        new node the head. This inserts at the front of the
+        linked list, which is O(1) since no traversal is needed.
+        """
+        new_node = Node(data)
+        new_node.next = self.head
+        self.head = new_node
+        self.size += 1
+
     def pop(self):
-        
+        """
+        Approach: Check if the stack is empty. If not, remove the head node
+        (the top of the stack) and return its data. Update the head to point
+        to the next node in the list.
+        """
+        if self.head is None:
+            return None
+        data = self.head.data
+        self.head = self.head.next
+        self.size -= 1
+        return data
+
 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..5049b3488 100644
--- a/Exercise_3.py
+++ b/Exercise_3.py
@@ -1,9 +1,29 @@
+"""
+Time complexity and space complexity for the following operations on a singly-linked list:
+
+1. Append
+    Time complexity: O(n) - we may need to traverse the entire list to find the end.
+    Space complexity: O(1) - we only allocate a new node, which does not depend on the current size of the list.
+
+2. Find
+    Time complexity: O(n) - we may need to traverse the entire list to find the key.
+    Space complexity: O(1) - we do not allocate any additional space that depends on the size of the list.
+
+3. Remove
+    Time complexity: O(n) - we may need to traverse the entire list to find the key to remove.
+    Space complexity: O(1) - we do not allocate any additional space that depends on the size of the list.
+
+"""
+
+
 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):
         """
@@ -16,17 +36,141 @@ def append(self, data):
         """
         Insert a new element at the end of the list.
         Takes O(n) time.
-        """
         
+        Approach: Create a new node with the given data. 
+        If the list is empty, set the head to this new node.
+        Otherwise, traverse to the end of the list and 
+        set the next pointer of the last node to the new node.
+        
+        """
+
+        new_node = ListNode(data)  # was: Node(data)
+        if self.head is None:
+            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.
+
+        Approach: Start from the head and traverse the list,
+        checking each node's data against the key. 
+        If a match is found, return that node. 
+        If the end of the list is reached without finding a match,
+        return None.
         """
-        
+        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.
+
+        Approach: Traverse the list while keeping track of the
+        previous node. If the current node's data matches the key,
+        update the previous node's next pointer to skip the current node.
+        If the node to remove is the head, update the head to the next node.
         """
+        current = self.head
+        previous = None
+        while current:
+            if current.data == key:
+                if previous is None:
+                    self.head = current.next
+                else:
+                    previous.next = current.next
+                return
+            previous = current
+            current = current.next
+
+def run_tests():
+    # --- Test append ---
+    ll = SinglyLinkedList()
+    ll.append(10)
+    ll.append(20)
+    ll.append(30)
+
+    values = []
+    current = ll.head
+    while current:
+        values.append(current.data)
+        current = current.next
+    assert values == [10, 20, 30], f"append failed: {values}"
+    print("append: OK ->", values)
+
+    # --- Test find (existing) ---
+    node = ll.find(20)
+    assert node is not None and node.data == 20, "find failed to locate existing key"
+    print("find existing: OK -> found", node.data)
+
+    # --- Test find (missing) ---
+    node = ll.find(999)
+    assert node is None, "find should return None for missing key"
+    print("find missing: OK -> None")
+
+    # --- Test remove head ---
+    ll.remove(10)
+    values = []
+    current = ll.head
+    while current:
+        values.append(current.data)
+        current = current.next
+    assert values == [20, 30], f"remove head failed: {values}"
+    print("remove head: OK ->", values)
+
+    # --- Test remove middle/tail ---
+    ll.append(40)
+    ll.remove(30)  # now middle-ish
+    values = []
+    current = ll.head
+    while current:
+        values.append(current.data)
+        current = current.next
+    assert values == [20, 40], f"remove middle failed: {values}"
+    print("remove middle: OK ->", values)
+
+    # --- Test remove tail ---
+    ll.remove(40)
+    values = []
+    current = ll.head
+    while current:
+        values.append(current.data)
+        current = current.next
+    assert values == [20], f"remove tail failed: {values}"
+    print("remove tail: OK ->", values)
+
+    # --- Test remove nonexistent key (should not raise, no-op) ---
+    ll.remove(999)
+    values = []
+    current = ll.head
+    while current:
+        values.append(current.data)
+        current = current.next
+    assert values == [20], f"remove nonexistent changed list: {values}"
+    print("remove nonexistent: OK -> no change", values)
+
+    # --- Test remove last remaining element ---
+    ll.remove(20)
+    assert ll.head is None, "list should be empty after removing last element"
+    print("remove last element: OK -> list is empty")
+
+    # --- Test operations on empty list ---
+    assert ll.find(1) is None, "find on empty list should return None"
+    ll.remove(1)  # should not raise
+    print("empty list operations: OK -> no crash")
+
+    print("\nAll tests passed!")
+
+
+run_tests()