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

Basic Implementation

Basic.java

import java.util.*;

public class Basic {
    static class Node {
        int value;
        Node left;
        Node right;
        Node(int value) { this.value = value; }
        Node(int value, Node left, Node right) { this.value = value; this.left = left; this.right = right; }
    }
    static String render(Node node) {
        if (node == null) return "_";
        if (node.left == null && node.right == null) return Integer.toString(node.value);
        return node.value + "(" + render(node.left) + "," + render(node.right) + ")";
    }
    static Node sampleTree() {
        return new Node(4, new Node(2, new Node(1), new Node(3)), new Node(6, new Node(5), new Node(7)));
    }
    static void preorder(Node node, List<Integer> output) { if (node == null) return; output.add(node.value); preorder(node.left, output); preorder(node.right, output); }
    public static void main(String[] args) { List<Integer> output = new ArrayList<>(); preorder(sampleTree(), output); System.out.println(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.