/**

* 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<Integer> preorderTraversal(TreeNode root) {

if(root == null) return Collections.emptyList();

List<Integer> result = new ArrayList<>();

Deque<TreeNode> st = new ArrayDeque<>();

st.push(root);

while(!st.isEmpty()) {

TreeNode t = st.pop();

result.add(t.val);

if(t.right != null) st.push(t.right);

if(t.left != null) st.push(t.left);

}

return result;

}

}