leetcode 144. Binary Tree Preorder Traversal ----- java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        
        Deque<TreeNode> queue = new ArrayDeque<TreeNode>();

        List list = new ArrayList<Integer>();

        if( root == null )
            return list;

        queue.add(root);

        while( !queue.isEmpty() ){

            TreeNode node = queue.poll();
            list.add(node.val);

            if( node.right != null )
                queue.addFirst(node.right);
            if( node.left != null )
                queue.addFirst(node.left);


        }


        return list;

        
    }
}