What is the difference between an array and a linked list?

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

def get(head, index):          # O(n) - must walk
    cur, i = head, 0
    while cur and i < index:
        cur, i = cur.next, i + 1
    return cur.val if cur else None

def push_front(head, val):     # O(1) - just relink
    return Node(val, head)


# --- demo ---
head = Node(10, Node(20, Node(30)))   # 10 -> 20 -> 30
print(get(head, 1))            # 20
head = push_front(head, 5)
print(get(head, 0))            # 5