144. 二叉树的前序遍历

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？

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Review:使用栈操作一波就可以

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Code:

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> res = new ArrayList<>();
        if(root==null) return res;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.empty()){
            TreeNode pop = stack.pop();
            res.add(pop.val);
            if (pop.right!=null) stack.push(pop.right);
            if (pop.left!=null) stack.push(pop.left);
        }
        return res;
    }
}