adding pre-course solutions

Exercise_1.py

@@ -1,24 +1,43 @@
+# 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), where n is the number of elements in the stack.
+
 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):
+        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')
 s.push('2')
 print(s.pop())
 print(s.show())
+

Exercise_2.py

@@ -1,32 +1,58 @@
+# Time Complexity:
+# push: O(1)
+# pop: O(1)
+#
+# 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.top = None
+
     def push(self, data):
-
+        new_node = Node(data)
+        new_node.next = self.top
+        self.top = new_node
+
     def pop(self):
-
+        if self.top is None:
+            return None
+
+        popped_value = self.top.data
+        self.top = self.top.next
+        return popped_value
+
+
 a_stack = Stack()
+
 while True:
-    #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"
+
+    if len(do) == 0:
+        continue
+
     operation = do[0].strip().lower()
+
     if operation == 'push':
         a_stack.push(int(do[1]))
+
     elif operation == 'pop':
         popped = a_stack.pop()
+
         if popped is None:
             print('Stack is empty.')
         else:
-            print('Popped value: ', int(popped))
+            print('Popped value:', int(popped))
+
     elif operation == 'quit':
         break
+

Exercise_3.py

@@ -1,13 +1,23 @@
+# Time Complexity:
+# append: O(n)
+# find: O(n)
+# remove: O(n)
+# Space Complexity: O(n), where n is the number of nodes in the linked list.
+
 class ListNode:
     """
-    A node in a singly-linked list.
+    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):
         """
-        Create a new singly-linked list.
+        Create a new singly linked list.
         Takes O(1) time.
         """
         self.head = None
@@ -17,16 +27,80 @@ def append(self, data):
         Insert a new element at the end of the list.
         Takes O(n) time.
         """
-
+        new_node = ListNode(data)
+
+        if self.head is None:
+            self.head = new_node
+            return
+
+        curr = self.head
+
+        while curr.next is not None:
+            curr = curr.next
+
+        curr.next = new_node
+
     def find(self, key):
         """
-        Search for the first element with `data` matching
-        `key`. Return the element or `None` if not found.
+        Search for the first element with data matching key.
+        Return the node or None if not found.
         Takes O(n) time.
         """
-
+        curr = self.head
+
+        while curr is not None:
+            if curr.data == key:
+                return curr
+            curr = curr.next
+
+        return None
+
     def remove(self, key):
         """
-        Remove the first occurrence of `key` in the list.
+        Remove the first occurrence of key in the list.
         Takes O(n) time.
         """
+        if self.head is None:
+            return False
+
+        if self.head.data == key:
+            self.head = self.head.next
+            return True
+
+        curr = self.head
+
+        while curr.next is not None:
+            if curr.next.data == key:
+                curr.next = curr.next.next
+                return True
+
+            curr = curr.next
+
+        return False
+
+    def show(self):
+        curr = self.head
+        result = []
+
+        while curr is not None:
+            result.append(curr.data)
+            curr = curr.next
+
+        return result
+
+
+ll = SinglyLinkedList()
+ll.append(10)
+ll.append(20)
+ll.append(30)
+
+print(ll.show())
+
+node = ll.find(20)
+if node is not None:
+    print("Found:", node.data)
+else:
+    print("Not found")
+
+ll.remove(20)
+print(ll.show())