Homework 1 exercises done

Exercise_1.py

@@ -1,21 +1,33 @@
+# Time Complexity: push/pop/peek/isEmpty/size -> O(1); show -> O(n)
+# Space Complexity: O(n), where n is the number of items stored
+
 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):
-
+  def __init__(self):
+    self._data = []
+
+  def isEmpty(self):
+    return len(self._data) == 0
+
+  def push(self, item):
+    self._data.append(item)
+
+  def pop(self):
+    if not self._data:
+      return None
+    return self._data.pop()
+
+  def peek(self):
+    if not self._data:
+      return None
+    return self._data[-1]
+
+  def size(self):
+    return len(self._data)
+
+  def show(self):
+    return list(self._data)
 
 s = myStack()
 s.push('1')

Exercise_2.py

@@ -1,16 +1,26 @@
 
+# Time Complexity: push/pop -> O(1) each
+# Space Complexity: O(n), where n is the number of nodes in the stack
 class Node:
     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.top is None:
+            return None
+        data = self.top.data
+        self.top = self.top.next
+        return data
 a_stack = Stack()
 while True:
     #Give input as string if getting an EOF error. Give input like "push 10" or "pop"

Exercise_3.py

@@ -1,8 +1,12 @@
+# Time Complexity: append/find/remove -> O(n)
+# Space Complexity: O(1) per operation; O(n) for n nodes stored
 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 +21,51 @@ 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
+        curr = self.head
+        while curr.next is not None:
+            curr = curr.next
+        curr.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.
         """
+        curr = self.head
+        while curr is not None:
+            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 is not None:
+            if curr.data == key:
+                if prev is None:
+                    self.head = curr.next
+                else:
+                    prev.next = curr.next
+                return
+            prev = curr
+            curr = curr.next
+
+if __name__ == "__main__":
+    sll = SinglyLinkedList()
+    sll.append(1)
+    sll.append(2)
+    sll.append(3)
+    found = sll.find(2)
+    print(found.data if found else None)
+    sll.remove(2)
+    print("Found" if sll.find(2) else "Not found")