import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定,请勿修改,直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    public int[] preorderTraversal(TreeNode root) {
        LinkedList<Integer> list = new LinkedList<>();
        if (root == null){
            return new int[0];
        }
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()){
            //处理根节点
            TreeNode node = stack.pop();
            list.add(node.val);

            //根左右,由于栈是先进后出,所以要让左节点后入栈,这样才能先弹出来
            if (node.right!=null){
                stack.push(node.right);
            }
            if (node.left != null){
                stack.push(node.left);
            }

        }
        return list.stream().mapToInt(Integer::intValue).toArray();
    }
}