144. 二叉树的前序遍历

题目：给你二叉树的根节点 root ，返回它节点值的 前序 遍历。
解法：迭代
思路：模拟
代码：
/**

• Definition for a binary tree node.

• public class TreeNode {

• int val;
  

• TreeNode left;
  

• TreeNode right;
  

• TreeNode() {}
  

• TreeNode(int val) { this.val = val; }
  

• TreeNode(int val, TreeNode left, TreeNode right) {
  

•     this.val = val;
  

•     this.left = left;
  

•     this.right = right;
  

• }
  

• }
  */
  class Solution {
  public List preorderTraversal(TreeNode root) {
  List res = new ArrayList();
  Stack stack = new Stack();
  if(root==null){
  return res;
  }
  stack.push(root);
  while(!stack.isEmpty()){
  TreeNode popNode = stack.pop();
  res.add(popNode.val);
  if(popNode.right!=null){
  stack.push(popNode.right);
  }
  if(popNode.left!=null){
  stack.push(popNode.left);
  }
  }
  return res;

  }
  }