Precourse-1

[shaikhmisrail]

No description provided.

[super30admin]

Let's evaluate each file one by one.

Exercise_1.py (Array-based Stack Implementation):

• Correctness: The implementation correctly uses a list to represent a stack. The methods (push, pop, peek, size, isEmpty, show) are implemented correctly for a stack. However, the show method returns the entire list, which might not be necessary and could expose internal representation.
• Time Complexity: The stated time complexities are correct: O(1) for push, pop, peek, and size. However, note that in Python, list.append() is amortized O(1), but worst-case can be O(n) due to resizing. Similarly, list.pop() is O(1) for the last element.
• Space Complexity: O(n) is correct as it stores n elements.
• Code Quality: The code is readable and well-structured. However, the isEmpty method can be simplified to return len(self.arr) == 0. Also, the peek method should handle the case when the stack is empty (currently it returns None, which is acceptable, but it might be better to raise an exception or document it). The show method might be better named as to_list or something that indicates it returns the entire stack as a list, but it's not a standard stack operation.
• Efficiency: The implementation is efficient. Using a list for a stack in Python is efficient due to the amortized constant time for append and pop.

Exercise_2.py (Linked List-based Stack Implementation):

• Correctness: The implementation uses a linked list with a dummy head node. The push and pop operations are correctly implemented. However, the pop method returns None when the stack is empty, which is acceptable, but the data type of the returned value might be inconsistent (e.g., if the stack contains integers, returning None might require the caller to handle it). Also, the Node class initializes with next = None, but in the Stack constructor, the head is initialized with a dummy node with data -1. This is acceptable, but the dummy node might not be necessary.
• Time Complexity: The stated time complexities are correct: O(1) for push and pop.
• Space Complexity: Each push operation allocates a new node, so the space complexity is O(n).
• Code Quality: The code is readable. However, the use of a dummy head node is not strictly necessary for a stack. Typically, we can just have a head pointer that points to the top node. Also, the pop method should handle the case when the stack is empty (it does, by returning None). The input handling part is provided and seems correct.
• Efficiency: The implementation is efficient. However, without a dummy node, the code could be simpler. For example, we can have self.head = None, and in push, we do: new_node = Node(data); new_node.next = self.head; self.head = new_node. Similarly, in pop, we check if self.head is None, then return None, else save self.head.data, set self.head = self.head.next, and return the data.

Exercise_3.py (Singly Linked List Implementation):

• Correctness:
  • The ListNode class has an issue: the init method has parameters data and next, but in the body, it only sets self.val = data and does not set the next pointer. This is incorrect. It should be:
    def init(self, data=None, next=None):
    self.data = data
    self.next = next
  • The append method: It traverses to the end of the list and appends a new node. This is correct but inefficient (O(n)).
  • The find method: It traverses the list to find a node with data equal to key. However, the loop does not advance curr (it is an infinite loop if the list is non-empty). It should be:
    while curr:
    if curr.data == key:
    return curr
    curr = curr.next
  • The remove method: It attempts to remove the first occurrence of key. However, it does not handle the case when the node to remove is the head. Also, the code does not set prev correctly initially (it starts with prev = None and curr = self.head.next, which is correct for a list with a dummy head). But note: the list has a dummy head, so the first element is self.head.next. The remove method should work correctly for non-head nodes, but if the node to remove is the first element (after dummy), then prev is None (because initially prev is None and curr is the first node) and we should update self.head.next to curr.next. However, in the code, if prev is None (meaning we are removing the first node), we do nothing? Actually, the code after the loop does:
    if prev:
    prev.next = curr.next
    return
    So if the node to remove is the first node (prev is None), we don't update any pointers. This is a bug.
• Time Complexity: The stated time complexities are correct for append (O(n)), find (O(n)), and remove (O(n)), but the implementation of find and remove has bugs.
• Space Complexity: O(n) for storing n nodes.
• Code Quality: The code has several issues. The ListNode class is incorrectly implemented. The linked list uses a dummy head, which is good, but the methods have bugs. Also, the method names and parameters are standard.
• Efficiency: The append method is O(n) which is acceptable for a singly linked list without a tail pointer. However, with a tail pointer we could make append O(1). But the problem does not require that.

Overall, the student shows understanding of data structures but has some implementation errors, especially in the linked list (Exercise_3.py). The stack implementations (Exercise_1 and Exercise_2) are mostly correct.

[super30admin]

For Exercise_1.py:

• Correctness: The implementation correctly uses a list to represent a stack. The push, pop, peek, size, and isEmpty methods are correctly implemented. The show method returns the entire list, which is acceptable for displaying the stack contents.
• Time Complexity: The stated time complexities are correct for all operations. However, note that in Python, list.append() and list.pop() are amortized O(1) operations, but worst-case can be O(n) due to resizing. This is acceptable for a stack.
• Space Complexity: The space complexity is O(n) as stated, which is correct.
• Code Quality: The code is clean and readable. However, the isEmpty method can be simplified to return len(self.arr) == 0. Also, the peek method should handle the case when the stack is empty (currently it returns None if empty, which is acceptable, but it might be better to raise an exception or document the behavior). The print statement in push might not be desirable in a production stack as it prints on every push.
• Efficiency: The implementation is efficient. Using a list for a stack is standard in Python.

For Exercise_2.py (linked list based stack):

• Correctness: The implementation uses a linked list with a dummy head node. The push and pop operations are correctly implemented. The pop method returns None when the stack is empty, which is acceptable.
• Time Complexity: The stated time complexities are correct: push and pop are O(1).
• Space Complexity: The space complexity is O(n) for n elements, which is correct.
• Code Quality: The code is readable. However, the Node class is defined with a data attribute, but in the Stack class, the head is initialized with a dummy node with data -1. This is acceptable. The pop method returns the data of the node, which is good. The input handling code is provided and works correctly.
• Efficiency: The implementation is efficient. Using a linked list avoids resizing issues.

For Exercise_3.py (singly linked list implementation):

• Correctness:
  • The ListNode class has an init method that sets val to data, but it does not set the next attribute. This is a bug. It should be self.val = data; self.next = next.
  • The SinglyLinkedList class has a dummy head node with value -1.
  • The append method: It traverses to the end of the list and appends the new node. This is correct but inefficient (O(n)).
  • The find method: It traverses the list to find the key. However, it uses curr.data but the node has attribute val (due to the ListNode definition). This will cause an AttributeError. Also, the loop does not advance curr - it is an infinite loop. It should be while curr: ... curr = curr.next.
  • The remove method: It tries to remove the first occurrence of key. It uses curr.data which should be curr.val. Also, it does not advance curr in the loop. The loop condition is while curr and curr.data != key but then it doesn't update curr inside the loop. This will cause an infinite loop. Also, the method does not handle the case when the node to remove is the first node (after dummy) correctly? Actually, because there is a dummy head, the prev should start as self.head. The current code sets prev to None initially and then updates. This might not work for the first node.
• Time Complexity: The stated time complexities are correct for append (O(n)), find (O(n)), and remove (O(n)), but the implementation is broken.
• Space Complexity: The space complexity is O(n), which is correct.
• Code Quality: The code has several bugs. The ListNode class is incorrectly defined. The find and remove methods have infinite loops. The attribute names are inconsistent (data vs val).
• Efficiency: Even if fixed, the append method is O(n). For a singly linked list, if we maintain a tail pointer, append can be O(1). But without that, it is acceptable.

Overall, the student shows understanding of data structures but has implementation errors in Exercise_3.py.