Linked List

Node + next

Each node holds a value and a reference to the next node:

[10|·]-> [20|·]-> [30|null]

Elements are scattered in memory. There's no index math, so reaching position k costs O(k), but splicing a node in or out is O(1).

Treasure hunt

Each clue tells you only where the next clue is. You can't jump to clue 5 — you follow the chain. But adding a clue means rewriting just two notes.

Insert at head, reverse

Reversing is the classic: walk once, flipping each next pointer backwards.

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

def reverse(head):
    prev = None
    while head:
        nxt = head.next   # save
        head.next = prev   # flip
        prev = head        # advance
        head = nxt
    return prev            # new head

Variants & patterns

Singly, doubly (has prev), and circular (tail points to head). Fast/slow pointers detect cycles and find the middle; a dummy head node removes edge cases when deleting.