Trees

Preorder Traversal

Visit the root before each subtree, producing root-left-right order.

Algorithm

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

Basic Implementation

basic.kt 
class Node(val value: Int, var left: Node? = null, var right: Node? = null)
fun render(node: Node?): String {
    if (node == null) return "_"
    if (node.left == null && node.right == null) return node.value.toString()
    return "${node.value}(${render(node.left)},${render(node.right)})"
}
fun sampleTree() = Node(4, Node(2, Node(1), Node(3)), Node(6, Node(5), Node(7)))
fun listString(values: List<Int>) = values.joinToString(", ", "[", "]")
fun preorder(node: Node?, output: MutableList<Int>) { if (node == null) return; output.add(node.value); preorder(node.left, output); preorder(node.right, output) }
fun main() { val output = mutableListOf<Int>(); preorder(sampleTree(), output); println(listString(output)) }

Complexity

  • Time: O(n)
  • Space: O(h) recursion stack

Implementation notes

  • Render tree structure explicitly instead of printing node objects.
  • The replay highlights the node, traversal state, queue, path, or search cursor that changes at each step.
preorder Preorder records the current node before visiting left and right subtrees.