Add PreCourse-1 PR submission

Exercise_1.py

@@ -1,21 +1,35 @@
 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):
-
+  # Please read sample.java file before starting.
+  # Kindly include Time and Space complexity at top of each file
+
+  # Time Complexity: O(1) for all operations
+  # Space Complexity: O(n) where n is the number of elements in the stack
+
+  def __init__(self):
+    self.stack = []
+
+  def isEmpty(self):
+    return len(self.stack) == 0
+
+  def push(self, item):
+    self.stack.append(item)
+
+  def pop(self):
+    if self.isEmpty():
+      return None
+    return self.stack.pop()
+
+  def peek(self):
+    if self.isEmpty():
+      return None
+    return self.stack[-1]
+
+  def size(self):
+    return len(self.stack)
+
+  def show(self):
+    return self.stack
+
 
 s = myStack()
 s.push('1')

Exercise_2.py

@@ -1,24 +1,37 @@
+# Time Complexity: O(1) for all operations
+# Space Complexity: O(n) where n is the number of elements in the stack
 
 class Node:
     def __init__(self, data):
-       self.data = data
-       self.next = None
-
+        self.data = data
+        self.next = None
+
+
 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
+        data = self.head.data
+        self.head = self.head.next
+        return data
+
+
 a_stack = Stack()
 while True:
-    #Give input as string if getting an EOF error. Give input like "push 10" or "pop"
+    # Give input as string if getting an EOF error. Give input like "push 10" or "pop"
     print('push <value>')
     print('pop')
     print('quit')
     do = input('What would you like to do? ').split()
-    #Give input as string if getting an EOF error. Give input like "push 10" or "pop"
+    # Give input as string if getting an EOF error. Give input like "push 10" or "pop"
     operation = do[0].strip().lower()
     if operation == 'push':
         a_stack.push(int(do[1]))

Exercise_3.py

@@ -1,9 +1,15 @@
+# Time Complexity: O(n) for append, find, and remove operations
+# Space Complexity: O(n) where n is the number of elements in 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):
         """
@@ -17,16 +23,44 @@ 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.
         """
+        if not self.head:
+            return
+
+        if self.head.data == key:
+            self.head = self.head.next
+            return
+
+        current = self.head
+        while current.next:
+            if current.next.data == key:
+                current.next = current.next.next
+                return
+            current = current.next
+        return None