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