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 List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (root == null) return result;
        Stack<TreeNode> stack1 = new Stack<TreeNode>();
        Stack<TreeNode> stack2 = new Stack<TreeNode>();
        int level = 0;
        stack1.push(root);
        while(!stack1.isEmpty() || !stack2.isEmpty()){
            List<Integer> row = new ArrayList<Integer>();
            if (!stack1.isEmpty()){
                while (!stack1.isEmpty()){
                    TreeNode node = stack1.pop();
                    row.add(node.val);
                    if (node.left!=null) stack2.push(node.left);
                    if (node.right!=null) stack2.push(node.right);
                }
            }
            else if (!stack2.isEmpty()){
                while (!stack2.isEmpty()){
                    TreeNode node = stack2.pop();
                    row.add(node.val);
                    if (node.right!=null) stack1.push(node.right);
                    if (node.left!=null) stack1.push(node.left);
                }
            }
            result.add(row);
        }
        return result;
    }
}