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 Swift DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.swift

final class Node {
    let value: Int
    var left: Node?
    var right: Node?
    init(_ value: Int, _ left: Node? = nil, _ right: Node? = nil) { self.value = value; self.left = left; self.right = right }
}
func render(_ node: Node?) -> String {
    guard let node = node else { return "_" }
    if node.left == nil && node.right == nil { return String(node.value) }
    return "\(node.value)(\(render(node.left)),\(render(node.right)))"
}
func sampleTree() -> Node {
    return Node(4, Node(2, Node(1), Node(3)), Node(6, Node(5), Node(7)))
}
func listString(_ values: [Int]) -> String { return "[" + values.map(String.init).joined(separator: ", ") + "]" }
func preorder(_ node: Node?, _ output: inout [Int]) { guard let node = node else { return }; output.append(node.value); preorder(node.left, &output); preorder(node.right, &output) }
var output: [Int] = []
preorder(sampleTree(), &output)
print(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.