Complete PreCourse-1

Exercise_1.py

@@ -1,20 +1,35 @@
 class myStack:
   #Please read sample.java file before starting.
   #Kindly include Time and Space complexity at top of each file
+  # Time Complexity : O(1) for all
+# Space Complexity : O(n) n is no of elements in the stack
+# Did this code successfully run on Leetcode :
+# Any problem you faced while coding this :
+
      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()

Exercise_2.py

@@ -1,3 +1,9 @@
+#Please read sample.java file before starting.
+#Kindly include Time and Space complexity at top of each file
+# Time Complexity : O(1) for push and pop
+# Space Complexity : O(n) where n is the number of elements in the stack
+# Did this code successfully run on Leetcode :
+# Any problem you faced while coding this :
 
 class Node:
     def __init__(self, data):
@@ -6,10 +12,19 @@ def __init__(self, data):
 
 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
+        popped = self.head.data
+        self.head = self.head.next
+        return popped
 
 a_stack = Stack()
 while True:

Exercise_3.py

@@ -1,8 +1,17 @@
+#Please read sample.java file before starting.
+#Kindly include Time and Space complexity at top of each file
+# Time Complexity : O(n) for append, find, and remove
+# Space Complexity : O(1) for all operations (excluding list storage)
+# Did this code successfully run on Leetcode :
+# Any problem you faced while coding this :
+
 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 +26,41 @@ 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
+        current = self.head
+        while current.next is not None:
+            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 is not None:
+            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 self.head is None:
+            return
+        if self.head.data == key:
+            self.head = self.head.next
+            return
+        current = self.head
+        while current.next is not None:
+            if current.next.data == key:
+                current.next = current.next.next
+                return
+            current = current.next