rambless

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) {
        if(root==null) {
            return new int[0];
        }
        List<Integer> list = new ArrayList<>();
        Deque<TreeNode> stack = new LinkedList<>();
        stack.push(root);
        TreeNode node;
        while(!stack.isEmpty()) {
            node = stack.pop();
            list.add(node.val);
            if(node.right!=null) {
                stack.push(node.right);
            }
            if(node.left!=null) {
                stack.push(node.left);
            }
        }
        int[] arr = new int[list.size()];
        for(int i=0; i<list.size(); i++) {
            arr[i] = list.get(i);
        }
        return arr;
    }
}