لیست پیوندی از صفر

class Node:
    def __init__(self, value, nxt=None):
        self.value = value
        self.next = nxt

class LinkedList:
    def __init__(self):
        self.head = None

    def append(self, value):
        if self.head is None:
            self.head = Node(value)
            return
        cur = self.head
        while cur.next is not None:
            cur = cur.next
        cur.next = Node(value)

    def find(self, value):
        cur = self.head
        while cur is not None:
            if cur.value == value:
                return cur
            cur = cur.next
        return None

ll = LinkedList()
for v in (10, 20, 30):
    ll.append(v)
print(ll.find(20).value)  # 20
print(ll.find(99))        # None