给定一个二叉树，返回它的 前序 遍历。

示例:

输入: [1,null,2,3]
1

2
/
3

输出: [1,2,3]

非递归实现

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> list = new ArrayList<>();
        if(root == null) return list;
        stack.push(root);

        while(!stack.isEmpty()){
            root = stack.pop();
            list.add(root.val);
            if(root.right != null){
                stack.push(root.right);
            }
            if(root.left != null){
                stack.push(root.left);
            }
        }
        return list;
    }
}