144. 二叉树的前序遍历(递归、迭代)2

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]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

法一：递归

//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) {
        ArrayList<Integer> pre = new ArrayList<>();
        preHelper(root,pre);
        return pre;
    }
    public void preHelper(TreeNode root, List<Integer> pre) {
        if(root==null) return;
        pre.add(root.val);
        preHelper(root.left,pre);
        preHelper(root.right,pre);
    }
}

法二：迭代

//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<Integer> res = new Stack<>();
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node != null) {
                res.add(node.val);
                stack.push(node.right);
                stack.push(node.left);
            }
        }
        return res;
    }
}