[Leetcode]-- Binary Tree Zigzag Level Order Traversal

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
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> curLvl = new Stack<TreeNode>();
        Stack<TreeNode> nextLvl = new Stack<TreeNode>();
        boolean leftToRight = true;
        curLvl.push(root);
        ArrayList<Integer> output = new ArrayList<Integer>();
        while(!curLvl.empty()){
            TreeNode curNode = curLvl.pop();
            output.add(curNode.val);
            if(leftToRight){
                if(curNode.left != null){
                    nextLvl.add(curNode.left);
                }
                if(curNode.right != null){
                    nextLvl.add(curNode.right);
                }
            } else{
                if(curNode.right != null){
                    nextLvl.add(curNode.right);
                }
                if(curNode.left != null){
                    nextLvl.add(curNode.left);
                }
            }
            if(curLvl.empty()){
                result.add(output);
                output = new ArrayList<Integer>();
                leftToRight = !leftToRight;
                curLvl.addAll(nextLvl);
                nextLvl.clear();
            }
        }
        return result;
    }
}