Exercise_3.py

# 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):
        """
        Create a new singly-linked list.
        Takes O(1) time.
        """
        self.head = None

    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")