Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1    \     2    /   3

return [1,2,3].

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {   public List<Integer> preorderTraversal(TreeNode root) {        Stack<TreeNode> nodeStack=new Stack<TreeNode>();        List<Integer> list=new ArrayList<Integer>();        TreeNode node=root;        while(!nodeStack.isEmpty() || node!=null){            if(node!=null){                list.add(node.val);                if(node.right!=null){                    nodeStack.add(node.right);                }                node=node.left;            }            else{            node=nodeStack.peek();            nodeStack.pop();            }        }        return list;    }}

思路：非递归的前序遍历