Build a Binary Tree

Create a fixed seven-node binary tree and render its shape.

Algorithm

The canonical tree is 4(2(1,3),6(5,7)), so this R DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.R

node <- function(value, left = NULL, right = NULL) list(value = value, left = left, right = right)
render <- function(n) {
  if (is.null(n)) return("_")
  if (is.null(n$left) && is.null(n$right)) return(as.character(n$value))
  paste0(n$value, "(", render(n$left), ",", render(n$right), ")")
}
sample_tree <- function() node(4, node(2, node(1), node(3)), node(6, node(5), node(7)))
list_string <- function(values) paste0("[", paste(values, collapse = ", "), "]")
cat(render(sample_tree()), "\n", sep = "")

Complexity

Time: O(n)
Space: O(n)

Implementation notes

node <- function(value, left = NULL, right = NULL) list(...) builds each tree node as an R list with value, left, and right fields.
sample_tree() directly nests the calls: node(4, node(2, node(1), node(3)), node(6, node(5), node(7))).
In that call, the second argument is the left child and the third argument is the right child. So node(2, node(1), node(3)) forms 2(1,3), and node(6, node(5), node(7)) forms 6(5,7).

Replay steps

left side:  1, 3 -> 2(1,3)
right side: 5, 7 -> 6(5,7)
root:       4 links both sides -> 4(2(1,3),6(5,7))

render() returns _ for NULL, as.character(n$value) for a leaf, and paste0(n$value, "(", render(n$left), ",", render(n$right), ")") for an internal node.
cat(render(sample_tree()), "\n", sep = "") prints exactly 4(2(1,3),6(5,7)).

node links A node stores one value plus references to its left and right children.