Complete Precourse 1

Exercise_1.py

@@ -1,24 +1,52 @@
+#Time Complexity: 
+# push: O(1)
+# pop: O(1)
+# peek: O(1)
+# isEmpty(): O(1)
+# size: O(1)
+# show: O(n)
+#Space Complexity: for all operations O(n) where n is the number of elements
+#Yes the code ran on Leetcode
+
 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):
-
+  def __init__(self):
+    """Initialize an empty stack using a Python list"""
+    self.stack = []
+
+
+  def isEmpty(self):
+    """If the stack is empty return True"""
+    return len(self.stack) == 0
+
+  def push(self, item):
+    """Add an element to the top of the stack"""
+    self.stack.append(item)
+
+  def pop(self):
+    """Remove an element from top of element and return the top element 
+    and return None if the stack is empty"""
+    if self.isEmpty()==True:
+      return None
+    return self.stack.pop()
+
 
-s = myStack()
-s.push('1')
-s.push('2')
-print(s.pop())
-print(s.show())
+  def peek(self):
+    """Return the top element of the stack without removing any element"""
+    return self.stack[-1]
+
+  def size(self):
+    """Return the length of the stack"""
+    return len(self.stack)
+
+  def show(self):
+    """Return the stack elements as a list (bottom → top)"""
+    return self.stack
+
+if __name__ == "__main__":
+  s = myStack()
+  s.push('1')
+  s.push('2')
+  print(s.pop())
+  print(s.show())

Exercise_2.py

@@ -1,15 +1,41 @@
+#Time Complexity: 
+# push: O(1)
+# pop: O(1)
+# peek: O(1)
+# isEmpty(): O(1)
+# size: O(1)
+# show: O(n)
+#Space Complexity: for all operations O(n) where n is the number of elements
+#Yes the code ran on Leetcode
 
 class Node:
+    """Node class for Linked List"""
     def __init__(self, data):
        self.data = data
        self.next = None
 
 class Stack:
     def __init__(self):
+        """Initialize an empty stack using a linked list"""
+        self.head = None
+        self._size = 0
 
     def push(self, data):
+        """Insert a new element on top of stack using linked list"""
+        new_node = Node(data)
+        new_node.next = self.head
+        self.head = new_node
+        self._size +=1
 
     def pop(self):
+        """Return the popped element from stack and return none if its empty"""
+        if self._size==0:
+            return None
+        popped = self.head.data
+        self.head = self.head.next
+        self._size -= 1
+        return popped
+
 
 a_stack = Stack()
 while True:

Exercise_3.py

@@ -3,6 +3,8 @@ class ListNode:
     A node in a singly-linked list.
     """
     def __init__(self, data=None, next=None):
+        self.data = data
+        self.next = None
 
 class SinglyLinkedList:
     def __init__(self):
@@ -17,16 +19,80 @@ 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: #if list is empty
+            self.head = new_node
+            return self.head
+        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
+        prev = None
+
+        #if key is head then remove it
+        if current and current.data == key:
+            self.head = current.next
+            current = None
+            return
+
+        #Traverse the list
+        while current and current.data != key:
+            prev = current
+            current = current.next
+
+        # If key not found
+        if current is None:
+            return
+
+        # Unlink the node
+        prev.next = current.next
+        current = None
+
+    def display(self):
+        """
+        Return all elements of the list as a Python list.
+        Takes O(n) time.
+        """
+        elements = []
+        current = self.head
+        while current:
+            elements.append(current.data)
+            current = current.next
+        return elements
+
+
+ll = SinglyLinkedList()
+ll.append(10)
+ll.append(20)
+ll.append(30)
+print("List:", ll.display())          # [10, 20, 30]
+
+node = ll.find(20)
+print("Found:", node.data if node else None)  # 20
+
+ll.remove(20)
+print("After removing 20:", ll.display())     # [10, 30]
+
+ll.remove(100)   # Key not in list
+print("After trying to remove 100:", ll.display())  # [10, 30]
+