Adding and Removing Nodes

Add at the front: O(1)

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

def show(head):
    out = []
    while head:
        out.append(str(head.value))
        head = head.next
    print(" -> ".join(out) or "(empty)")

head = Node(2, Node(3))
head = Node(1, head)          # new node points at the old head
show(head)

Output

1 -> 2 -> 3

Add at the end: O(n)

You have to walk to the last node first (unless you also keep a tail pointer):

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

def append(head, value):
    if head is None:
        return Node(value)
    current = head
    while current.next:
        current = current.next
    current.next = Node(value)
    return head

head = None
for v in [5, 6, 7]:
    head = append(head, v)
node = head
while node:
    print(node.value, end=" ")
    node = node.next
print()

Output

5 6 7 

Remove a value

To remove a node, make the node before it skip over it. A dummy node in front makes removing the head work the same way as removing any other node:

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

def remove_all(head, target):
    dummy = Node(0, head)
    prev = dummy
    while prev.next:
        if prev.next.value == target:
            prev.next = prev.next.next     # skip it
        else:
            prev = prev.next
    return dummy.next

head = Node(7, Node(1, Node(7, Node(2, Node(7)))))
head = remove_all(head, 7)
while head:
    print(head.value)
    head = head.next

Output

1
2

Think of it like this: Removing someone from a human chain: the person before them lets go and grabs the hand of the person after them.

Common mistake: Always check for None before using .next. None.next crashes with an AttributeError.