Precourse-1 completed

Exercise_1.py

@@ -1,24 +1,38 @@
+# Time Complexity : O(1) for isEmpty, push, pop, peek, size
+# Space Complexity : O(n) for storing elements in 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):
-
+    # Please read sample.java file before starting.
+    # Kindly include Time and Space complexity at top of each file
+    def __init__(self):
+        self.stack = []
+        self.count = 0
+
+    def isEmpty(self):
+        return self.count == 0
+
+    def push(self, item):
+        self.stack.append(item)
+        self.count += 1
+
+    def pop(self):
+        if self.count >= 1:
+            self.count -= 1
+            return self.stack.pop()
+        else:
+            return None
+
+    def peek(self):
+        return self.stack[-1] if not self.isEmpty() else None
+
+    def size(self):
+        return self.count
+
+    def show(self):
+        return self.stack
+
 
 s = myStack()
-s.push('1')
-s.push('2')
+s.push("1")
+s.push("2")
 print(s.pop())
 print(s.show())

Exercise_2.py

@@ -1,32 +1,45 @@
-
+# The intution here is like inserting in reverese order
+# Time Complexity : O(1) for push, O(n) for pop
+# Space Complexity : O(1) for storing n nodes in 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.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            # Stack is empty
+       pop_value = self.head.data # Store the value to return
+       self.head = self.head.next   # Move the top pointer down
+       return pop_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"
+    # 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"
     operation = do[0].strip().lower()
-    if operation == 'push':
+    if operation == "push":
         a_stack.push(int(do[1]))
-    elif operation == 'pop':
+    elif operation == "pop":
         popped = a_stack.pop()
         if popped is None:
-            print('Stack is empty.')
+            print("Stack is empty.")
         else:
-            print('Popped value: ', int(popped))
-    elif operation == 'quit':
+            print("Popped value: ", int(popped))
+    elif operation == "quit":
         break

Exercise_3.py

@@ -2,31 +2,63 @@ 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.
         """
-
+        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.next
+        while curr:
+            if curr.data == key:
+                return curr.data
+            curr = curr.next
+        return None
+
     def remove(self, key):
         """
         Remove the first occurrence of `key` in the list.
         Takes O(n) time.
         """
+
+        curr = self.head.next
+        while curr.next:
+            if curr.next.data == key:
+                curr.next = curr.next.next
+                return
+            curr = curr.next
+
+
+sl = SinglyLinkedList()
+for i in range(5):
+    sl.append(i)
+sl.remove(3)
+sl.append(7)
+print("Found key {}".format(sl.find(2)))
+sl.remove(2)
+if sl.find(2) is None:
+    print("Not found")