32 - III. 从上到下打印二叉树 III

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> lists = new ArrayList<>();
        if(root == null) return lists;
        Stack<TreeNode> s1 = new Stack<>();
        Stack<TreeNode> s2 = new Stack<>();
        boolean lToR = true;
        s1.push(root);
        while(!s1.isEmpty() || !s2.isEmpty()){
            List<Integer> list = new ArrayList<>();
            if(lToR){
                while(!s1.isEmpty()){
                    TreeNode node = s1.pop();
                    list.add(node.val);
                    if(node.left != null) s2.push(node.left);
                    if(node.right != null) s2.push(node.right);
                }
            }else{
                while(!s2.isEmpty()){
                    TreeNode node = s2.pop();
                    list.add(node.val);
　　　　　　　　　　　　//先添加右节点
                    if(node.right != null) s1.push(node.right);
                    if(node.left != null) s1.push(node.left);
                }                   
            }
            lToR = !lToR;
            lists.add(list);       
        }
        return lists;
    }
}