recursive version

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */class Solution {    List
    
      ret = new ArrayList<>();    public List
     
       preorderTraversal(TreeNode root) {        dfs(root);        return ret;    }        public void dfs(TreeNode node) {        if (node == null) return;        ret.add(node.val);        dfs(node.left);        dfs(node.right);    }}
     
    

Non-recursive. It's relatively simple. So no explaination.

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