import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;

public class Solution144 {
    /**
     * 前序遍历
     *
     * @param root 根节点
     * @return 遍历数组
     */
    public List<Integer> preorderTraversal(TreeNode root) {
        if (root == null) {
            return new ArrayList<>();
        }
        List<Integer> list = new ArrayList<>();
        list.add(root.val);
        list.addAll(preorderTraversal(root.left));
        list.addAll(preorderTraversal(root.right));
        return list;
    }

    /**
     * 模拟栈
     * @param root 根节点
     * @return 数组
     */
    public List<Integer> preorderTraversal2(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }

        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode node = root;
        while (!stack.isEmpty() || node != null) {
            while (node != null) {
                res.add(node.val);
                stack.push(node);
                node = node.left;
            }
            node = stack.pop();
            node = node.right;
        }
        return res;
    }

    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode() {
        }

        TreeNode(int val) {
            this.val = val;
        }

        TreeNode(int val, TreeNode left, TreeNode right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }
}