Solution32-3.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>> levelOrder(TreeNode root) {
        List<List<Integer>> rowList = new ArrayList<>();
        if (root == null) {
            return rowList;
        }

        Stack<TreeNode> stack1 = new Stack<>();
        Stack<TreeNode> stack2 = new Stack<>();

        stack1.push(root);

        while (!stack1.isEmpty() || !stack2.isEmpty()) {
            if (!stack1.isEmpty()) {
                List<Integer> colList = new ArrayList<>();
                while (!stack1.isEmpty()) {
                    TreeNode node = stack1.pop();
                    colList.add(node.val);
                    if (node.left != null) {
                        stack2.push(node.left);
                    }
                    if (node.right != null) {
                        stack2.push(node.right);
                    }
                }
                rowList.add(colList);
            } else {
                List<Integer> colList = new ArrayList<>();
                while (!stack2.isEmpty()) {
                    TreeNode node = stack2.pop();
                    colList.add(node.val);
                    if (node.right != null) {
                        stack1.push(node.right);
                    }
                    if (node.left != null) {
                        stack1.push(node.left);
                    }
                }
                rowList.add(colList);
            }
        }

        return rowList;
    }
}