Exercise_3.py

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.
        """
        if not self.head:
            self.head = ListNode(data=data)
            return
        curr = self.head
        while curr.next:
            curr = curr.next
        curr.next = ListNode(data=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.
        """
        current = self.head 
        while current != None: 
            if current.data == key: 
                return True   
            current = current.next  
        return False

    def remove(self, key):
        """
        Remove the first occurrence of `key` in the list.
        Takes O(n) time.
        """
        if self.head is None:
            return

        if self.head.data == key:
            self.head = self.head.next
            return

        itr = self.head
        while itr.next:
            if itr.next.data == key:
                itr.next = itr.next.next
                break
            itr = itr.next