Solved precourse 1

Exercise_1.py

@@ -1,20 +1,30 @@
 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):
+    # Please read sample.java file before starting.
+    # Kindly include Time and Space complexity at top of each file
+    def __init__(self):
+        self.st = []
 
+    def isEmpty(self):
+        return len(self.st) == 0
 
-     def peek(self):
+    def push(self, item):
+        self.st.append(item)
 
-     def size(self):
-
-     def show(self):
+    def pop(self):
+        if self.isEmpty():
+            return
+        return self.st.pop()
+
+    def peek(self):
+        if self.isEmpty():
+            return None
+        return self.st[-1]
+
+    def size(self):
+        return len(self.st)
+
+    def show(self):
+        return self.st
 
 
 s = myStack()

Exercise_2.py

@@ -6,11 +6,23 @@ def __init__(self, data):
 
 class Stack:
     def __init__(self):
+        self.top = None
 
     def push(self, data):
+        newNode = Node(data)
+        if self.top is None:
+            self.top = newNode
+            return
+        newNode.next = self.top
+        self.top = newNode
 
     def pop(self):
-
+        if self.top is None:
+            return None
+        popped = self.top.data
+        self.top = self.top.next
+        return popped
+
 a_stack = Stack()
 while True:
     #Give input as string if getting an EOF error. Give input like "push 10" or "pop"

Exercise_3.py

@@ -3,6 +3,8 @@ 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 +19,44 @@ def append(self, data):
         Insert a new element at the end of the list.
         Takes O(n) time.
         """
+        if self.head is None:
+            self.head = ListNode(data)
+            return
+        curr = self.head
+        while curr.next:
+            curr = curr.next
+        curr.next = ListNode(data)
 
     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.
         """
+        curr = self.head
+        while curr:
+            if curr.data == key:
+                return curr
+            curr = curr.next
+        return None
 
     def remove(self, key):
         """
         Remove the first occurrence of `key` in the list.
         Takes O(n) time.
         """
+        prev = None
+        curr = self.head
+
+        while curr:
+            if curr.data == key:
+                if prev is None:
+                    self.head = curr.next
+                else:
+                    prev.next = curr.next
+                return True
+            prev = curr
+            curr = curr.next
+
+        return False
+