FlippingTreeTraversal.java

package com.jadekler.app;

import java.util.Stack;


/**
 * Given a tree of the form:
 *      1
 *    2   3
 *   4 5 6 7
 *
 * Emit 1 3 2 4 5 6 7
 */
public class FlippingTreeTraversal {
    public Node root;

    public class Node {
        public int val;
        public Node left;
        public Node right;

        public Node(int val) {
            this.val = val;
        }
    }

    public static void main( String[] args ) {
        FlippingTreeTraversal treeTraverser = new FlippingTreeTraversal();
        treeTraverser.traverse();
    }

    public FlippingTreeTraversal() {
        Node root = new Node(1);
        root.left = new Node(2);
        root.right = new Node(3);
        root.left.left = new Node(4);
        root.left.right = new Node(5);
        root.right.left = new Node(6);
        root.right.right = new Node(7);

        this.root = root;
    }

    /**
     * The general idea is that we go left-to-right for the left stack (putting children right-to-left on the
     * second stack), then we go right-to-left for the right stack (putting children left-to-right on the 
     * first stack), then we go left-to-right for the left stack, etc.
    **/
    public void traverse() {
        Stack leftStack = new Stack();
        Stack rightStack = new Stack();

        leftStack.push(this.root);

        while (!leftStack.isEmpty()) {
            Node curNode = (Node)leftStack.pop();

            rightStack.push(curNode.left);
            rightStack.push(curNode.right);
            System.out.println("left: "+curNode.val);

            if (leftStack.isEmpty() && !rightStack.isEmpty()) {
                while (!rightStack.isEmpty()) {
                    Node otherDirectionNode = (Node)rightStack.pop();

                    if (otherDirectionNode != null) {
                        System.out.println("right: "+otherDirectionNode.val);
                        leftStack.push(otherDirectionNode.right);
                        leftStack.push(otherDirectionNode.left);
                    }
                }
            }
        }
    }
}