Completed PreCourse-1 : Exercise 1, 2, and 3

Exercise_1.py

@@ -1,20 +1,36 @@
+# Time Complexity : O(1)
+# Space Complexity : O(n)
+# Did this code successfully run on Leetcode : Yes
+# Any problem you faced while coding this : 
 class myStack:
   #Please read sample.java file before starting.
   #Kindly include Time and Space complexity at top of each file
      def __init__(self):
-
+         self.s = []
+         self.i = -1
+
      def isEmpty(self):
+          return len(self.s) == 0
 
      def push(self, item):
+          self.s.append(item)
+          self.i += 1
 
      def pop(self):
-
+        if self.i >= 0:
+          a = self.s.pop()
+          self.i -= 1
+          return a
 
      def peek(self):
-
+         return self.s[self.i]
+
      def size(self):
+         return len(self.s)
 
      def show(self):
+         for e in self.s:
+             print(e)
 
 
 s = myStack()

Exercise_2.py

@@ -1,15 +1,43 @@
-
+# Time Complexity : O(1)
+# Space Complexity : O(n)
+# Did this code successfully run on Leetcode : Yes
+# Any problem you faced while coding this : 
 class Node:
     def __init__(self, data):
        self.data = data
        self.next = None
 
 class Stack:
     def __init__(self):
+        self.head = Node(None)
+        self.tail = self.head
+        self.count = 0
 
     def push(self, data):
+        newNode = Node(data)
+        if self.tail == self.head:
+            self.head.next = newNode
+        else:
+            self.tail.next = newNode
+
+        self.tail = newNode
+        self.count += 1
+
 
     def pop(self):
+        if self.count > 0:
+            data = self.tail.data
+            temp = self.head
+            while temp.next != self.tail:
+                temp = temp.next
+
+            self.tail = temp
+            self.tail.next = None
+            self.count -= 1
+
+            return data
+        else:
+            return None
 
 a_stack = Stack()
 while True:

Exercise_3.py

@@ -3,30 +3,73 @@ 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):
         """
         Create a new singly-linked list.
         Takes O(1) time.
         """
-        self.head = None
+        self.head = ListNode()
 
     def append(self, data):
         """
         Insert a new element at the end of the list.
         Takes O(n) time.
         """
+        newNode = ListNode(data)
+        temp = self.head
+        while temp.next != None:
+            temp = temp.next
+
+        temp.next = newNode
+
 
     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.
         """
+        temp = self.head
+
+        while temp.next:
+            if temp.next.data == key:
+                return temp.next
+            else:
+                temp = temp.next
 
+        return None
+
+
     def remove(self, key):
         """
         Remove the first occurrence of `key` in the list.
         Takes O(n) time.
         """
+        previous = self.head
+
+        while previous.next:
+            if previous.next.data == key:
+                break
+            else:
+                previous = previous.next
+
+        # if we didn't find the key, previous will be TAIL, and previous.next will be None
+        # else previous.next will be the item to remove
+        if previous.next:
+            itemToRemove = previous.next
+            previous.next = itemToRemove.next
+            itemToRemove.next = None
+
+
+ll = SinglyLinkedList()
+ll.append(1)
+ll.append(2)
+ll.append(3)
+print((ll.find(2)).data)
+print(ll.find(5))
+ll.remove(2)
+print(ll.find(2))