Completed PreCourse 1

.gitignore

@@ -0,0 +1 @@
+.DS_Store

Exercise_1.py

@@ -1,21 +1,29 @@
 class myStack:
-  #Please read sample.java file before starting.
-  #Kindly include Time and Space complexity at top of each file
+  #Time Complexity: push: O(1), pop: O(1), peek: O(1), isEmpty: O(1), size: O(1), show: O(n)
+  #Space Complexity: O(n)
+
      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()
+
      def peek(self):
-
+         if not self.isEmpty():
+             return self.items[-1]
+
      def size(self):
+         return len(self.items)
 
      def show(self):
-
+         print(self.items)
 
 s = myStack()
 s.push('1')

Exercise_2.py

@@ -1,15 +1,28 @@
 
 class Node:
+#TIME: push: O(1), pop: O(1), __init__ : O(1)
+#SPACE: O(n)
     def __init__(self, data):
        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.isEmpty():
+            return None
+        popped_node = self.top
+        self.top = self.top.next
+        return popped_node.data
+
+
 
 a_stack = Stack()
 while True:

Exercise_3.py

@@ -1,8 +1,12 @@
 class ListNode:
+    #TIME: append: O(n), find: O(n), remove: O(n)
+    #SPACE: O(1)
     """
     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):
@@ -13,20 +17,63 @@ def __init__(self):
         self.head = None
 
     def append(self, data):
+        new_node = ListNode(data)
+        # If list is empty
+        if self.head is None:
+            self.head = new_node
+            return
+        # Traverse to the end of the list
+        current = self.head
+        while current.next:
+            current = current.next
+        current.next = new_node
+
         """
         Insert a new element at the end of the list.
         Takes O(n) time.
         """
 
     def find(self, key):
+        current = self.head
+        while current:
+            if current.data == key:
+                return current
+            current = current.next
+        return None
         """
         Search for the first element with `data` matching
         `key`. Return the element or `None` if not found.
         Takes O(n) time.
         """
 
     def remove(self, key):
+        current = self.head
+        previous = None
+
+        #if head contains the key
+        if current and current.data == key:
+            self.head = current.next
+            return
+        #search for the key to be deleted
+        while current and current.data != key:
+            previous = current
+            current = current.next
+        #if key was not present in linked list
+        if current is None:
+            return
+        #unlink the node from linked list
+        previous.next = current.next
         """
         Remove the first occurrence of `key` in the list.
         Takes O(n) time.
         """
+
+
+
+ll = SinglyLinkedList() 
+ll.append(10) 
+ll.append(20) 
+ll.append(30) 
+print(ll.find(20).data) # 20 
+ll.remove(20) 
+print(ll.find(20)) # None