• java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if(root == null)
            return res;
        Stack<TreeNode> odd = new Stack<TreeNode>();
        Stack<TreeNode> even = new Stack<TreeNode>();
        odd.push(root);
        while(!odd.isEmpty() || !even.isEmpty()){
            List<Integer> tem = new ArrayList<Integer>();
            while(!odd.isEmpty()){
                TreeNode t = odd.pop();
                tem.add(t.val);
                if(t.left!=null)
                    even.push(t.left);
                if(t.right!=null)
                    even.push(t.right);
            }
            res.add(tem);
            tem = new ArrayList<Integer>();
            while(!even.isEmpty()){
                TreeNode t = even.pop();
                tem.add(t.val);
                if(t.right!=null)
                    odd.push(t.right);
                if(t.left!=null)
                    odd.push(t.left);
            }
            if(!tem.isEmpty())
                res.add(tem);
        }
        return res;
    }
}