Insert at Head

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

Algorithm

The replay labels nodes by value, such as node(20), and never exposes object identity or memory addresses. This R DSA implementation uses the same small chain as the rest of the DSA track.

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 = "")

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.