从左到右和从右到左交替打印二叉树的级别顺序遍历

Question

This is an interview question.
We want to print binary tree by it levels, but with some change:
At even levels, the printing will be from left to right.
At odd levels, the printing will be from right to left.

Example: the output for the following tree will be 1 3 2 4 7 :

I tried to use the code here (normal level order traversal, method 2) just with a few changes of keeping the level (for knowing if to print from left to right or from right to left) and of course adding relevant condition for printing in the correct direction.

Unfortunately, my code below doesn't work well on trees that are not small - I have problem of understanding how to store the nodes in correct order in the loop. please tell me how can I fix it:

Auxiliary class:

public class Node {
    int data; 
    Node left, right; 

    public Node(int item) { 
        data = item; 
        left = null; 
        right = null; 
    } 
}                 

Main class:

public class BinaryTree {

    class LevelNode {
        int level;
        Node node;

        public LevelNode(int level, Node node) {
            this.level = level;
            this.node = node;
        }
    };

    private Node root; 

    void printLevelOrder(){ 
        int level = 0;
        Queue<LevelNode> queue = new LinkedList<LevelNode>();

        queue.add(new LevelNode(level, root)); 
        while (!queue.isEmpty()){ 

            LevelNode tempNode = queue.poll();
            level = tempNode.level;
            System.out.print(tempNode.node.data + " "); 

            if ( (level & 1) == 1 ) {
                if (tempNode.node.left != null) { 
                    queue.add(new LevelNode(level + 1, tempNode.node.left)); 
                } 
                if (tempNode.node.right != null) { 
                    queue.add(new LevelNode(level + 1, tempNode.node.right));
                }
            }
            else {
                if (tempNode.node.right != null) { 
                    queue.add(new LevelNode(level + 1, tempNode.node.right));
                }
                if (tempNode.node.left != null) { 
                    queue.add(new LevelNode(level + 1, tempNode.node.left)); 
                } 
            }
        } 
    }
 }                

And for the example above, my code prints: 1 3 2 7 4
Here is the main method which generates the output:

public static void main (String[] args) {
   BinaryTree tree_level = new BinaryTree(); 
   tree_level.root = new Node(1); 
   tree_level.root.left = new Node(2); 
   tree_level.root.right = new Node(3); 
   tree_level.root.left.left = new Node(4); 
   tree_level.root.right.right = new Node(7); 
   tree_level.printLevelOrder(); 
}

Answer 1

In my opinion it should be easier to accomplish the task step-by-step, ie

1. Processing the current level
2. Preparing the next level in the right order.

In this case you shouldn't need the LevelNode class in order to store the level, because the level is known when you process the level.

void printLevelOrderFixed() {
    List<Node> currLevel = new ArrayList<>();
    currLevel.add(root);

    int level = 1;
    while(currLevel.size() > 0) {

        // Output
        currLevel.forEach(x -> System.out.print(x + " "));

        // Preparation for next level
        List<Node> nextLevel = new ArrayList<>();
        for (int i = currLevel.size() - 1; i >= 0; i--) {
            Node left = currLevel.get(i).left;
            Node right = currLevel.get(i).right;

            if (level % 2 == 0) {
                if (left != null) nextLevel.add(left);
                if (right != null) nextLevel.add(right);                    
            } else {
                if (right != null) nextLevel.add(right);    
                if (left != null) nextLevel.add(left);
            }
        }
        currLevel.clear();
        currLevel.addAll(nextLevel);

        level++;
    }
    System.out.println("");
}

Extended test driver with your result and the fixed result:

public static void main(String[] args) {
    System.out.println("Example 1. Expected output: 1 3 2 4 7 ");

    BinaryTree tree_level = new BinaryTree();
    tree_level.root = new Node(1);
    tree_level.root.left = new Node(2);
    tree_level.root.right = new Node(3);
    tree_level.root.left.left = new Node(4);
    tree_level.root.right.right = new Node(7);

    tree_level.printLevelOrder();
    System.out.println();
    tree_level.printLevelOrderFixed();
    System.out.println();

    System.out.println("Example 2. Expected output: 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 ");
    /*             1
     *         3       2
     *       4   5   6   7
     *      5 4 3 2 1 0 9 8
     *     6               7
     *    9                 8
     */
    BinaryTree tree_level2 = new BinaryTree();
    tree_level2.root = new Node(1);

    tree_level2.root.left = new Node(3);
    tree_level2.root.right = new Node(2);

    tree_level2.root.left.left = new Node(4);
    tree_level2.root.left.right = new Node(5);
    tree_level2.root.right.left = new Node(6);
    tree_level2.root.right.right = new Node(7);

    tree_level2.root.left.left.left = new Node(5);
    tree_level2.root.left.left.right = new Node(4);
    tree_level2.root.left.right.left = new Node(3);
    tree_level2.root.left.right.right = new Node(2);
    tree_level2.root.right.left.left = new Node(1);
    tree_level2.root.right.left.right = new Node(0);
    tree_level2.root.right.right.left = new Node(9);
    tree_level2.root.right.right.right = new Node(8);

    tree_level2.root.left.left.left.left = new Node(6);
    tree_level2.root.right.right.right.right = new Node(7);
    tree_level2.root.left.left.left.left.left = new Node(9);
    tree_level2.root.right.right.right.right.right = new Node(8);

    tree_level2.printLevelOrder();
    System.out.println();
    tree_level2.printLevelOrderFixed();
    System.out.println();
}

Output of the test driver:

Example 1. Expected output: 1 3 2 4 7 
1 3 2 7 4 
1 3 2 4 7 

Example 2. Expected output: 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 
1 2 3 6 7 4 5 0 1 8 9 4 5 2 3 7 6 8 9 
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9