Insert at Head

Insert a new first node by pointing it at the old head and then moving the head pointer.

Algorithm

Basic Implementation

basic.R

node <- function(value, next_node = NULL) {
  list(value = value, nxt = next_node)
}

render <- function(head) {
  parts <- c()
  cursor <- head
  while (!is.null(cursor)) {
    parts <- c(parts, as.character(cursor$value))
    cursor <- cursor$nxt
  }
  paste0(paste(parts, collapse = " -> "), " -> null")
}

head <- node(20, node(30))
new_head <- node(10)
new_head$nxt <- head
head <- new_head
cat(render(head), "\n", sep = "")

Head insertion changes only two references: the new node points at the old head, then head moves to the new node.

Step 1 - Old first node

Before insertion, head points at node(20).

Step 2 - New node links to old head

Set new.next to the old first node before moving head.

Step 3 - Head moves to the new node

The final chain has 10 first: 10 -> 20 -> 30 -> null.

Complexity

Time: O(1)
Space: O(1)

Implementation notes

Keep the explicit node and pointer/reference operations; array shortcuts hide the linked-list state this lesson is meant to replay.
The final output prints the chain in a deterministic a -> b -> null form for cross-language comparison.

old head The previous first node becomes the second node.

constant-time insert Only the new node and head pointer change.