144. Binary Tree Preorder Traversal

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 class Solution {
11     public List<Integer> preorderTraversal(TreeNode root) {
12         List<Integer> res = new ArrayList<>() ;
13         Stack<TreeNode> stack = new Stack<>() ;
14         stack.push(root) ;
15         while(!stack.isEmpty()){
16             TreeNode node = stack.pop() ;
17             if (node == null){
18                 continue ;
19             }
20             res.add(node.val) ;
21             stack.push(node.right) ;
22             stack.push(node.left) ;
23         }
24         return res ;
25     }
26 }