leetcode 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]

/**
 * 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) {
        
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        if (root == null)return list;
        stack.push(root);

        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            list.add(node.val);
            if (node.right != null)
                stack.push(node.right);
            if (node.left != null)
                stack.push(node.left);    

        } 
        return list;
    }
}