Level-Order Traversal

Visit a tree breadth-first with a queue.

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)));
    }
    public static void main(String[] args) { Queue<Node> queue = new ArrayDeque<>(); queue.add(sampleTree()); List<Integer> output = new ArrayList<>(); while (!queue.isEmpty()) { Node node = queue.remove(); output.add(node.value); if (node.left != null) queue.add(node.left); if (node.right != null) queue.add(node.right); } System.out.println(output); }
}

Complexity

Time: O(n)
Space: O(w) queue space

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.

level order Level-order traversal uses a queue to visit shallower nodes first.