Need some help with my Java code

  • 0

    Should be a simple Stack solution.
    Anyway it can run faster though? (it beats 28.88% currently)

    public List<Integer> preorderTraversal(TreeNode root) {
            if (root == null) return Collections.emptyList(); 
            List<Integer> ret = new ArrayList<>();       
            Stack<TreeNode> stack = new Stack<>();
            while (!stack.empty()){
                TreeNode curr = stack.pop(); 
                if (curr.right != null) stack.push(curr.right); 
                if (curr.left != null) stack.push(curr.left); 
            return ret; 

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