Binary Tree Level Order Traversal

Binary Tree Level Order Traversal: http://www.lintcode.com/problem/binary-tree-level-order-traversal/

// version 3: BFS. two queues
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Level order a list of lists of integer
     */
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if (root == null) {
            return result;
        }

        ArrayList<TreeNode> Q1 = new ArrayList<TreeNode>();
        ArrayList<TreeNode> Q2 = new ArrayList<TreeNode>();

        Q1.add(root);
        while (Q1.size() != 0) {
            ArrayList<Integer> level = new ArrayList<Integer>();
            Q2.clear();
            for (int i = 0; i < Q1.size(); i++) {
                TreeNode node = Q1.get(i);
                level.add(node.val);
                if (node.left != null) {
                    Q2.add(node.left);
                }
                if (node.right != null) {
                    Q2.add(node.right);
                }
            }

            // swap q1 and q2
            ArrayList<TreeNode> temp = Q1;
            Q1 = Q2;
            Q2 = temp;

            // add to result
            result.add(level);
        }

        return result;
    }
}

Binary Tree Zigzag Level Order Traversal: http://www.lintcode.com/problem/binary-tree-zigzag-level-order-traversal/

public class Solution {
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();

        if (root == null) {
            return result;
        }

        Stack<TreeNode> currLevel = new Stack<TreeNode>();
        Stack<TreeNode> nextLevel = new Stack<TreeNode>();
        Stack<TreeNode> tmp;

        currLevel.push(root);
        boolean normalOrder = true;

        while (!currLevel.isEmpty()) {
            ArrayList<Integer> currLevelResult = new ArrayList<Integer>();

            while (!currLevel.isEmpty()) {
                TreeNode node = currLevel.pop();
                currLevelResult.add(node.val);

                if (normalOrder) {
                    if (node.left != null) {
                        nextLevel.push(node.left);
                    }
                    if (node.right != null) {
                        nextLevel.push(node.right);
                    }
                } else {
                    if (node.right != null) {
                        nextLevel.push(node.right);
                    }
                    if (node.left != null) {
                        nextLevel.push(node.left);
                    }
                }
            }

            result.add(currLevelResult);
            tmp = currLevel;
            currLevel = nextLevel;
            nextLevel = tmp;
            normalOrder = !normalOrder;
        }

        return result;

    }
}