preOrderTreeNodeStack.java

public class preOrderTreeNodeStack {
    public ArrayList<Object> preorderTraversal(TreeNode root) {
        ArrayList<Object> returnList = new ArrayList<Object>();
 
        if(root == null)
            return returnList;
 
        Deque<TreeNode> stack = new ArrayDeque<TreeNode>();
        stack.push(root);
 
        while(!stack.isEmpty()){
            TreeNode n = stack.pop();
            returnList.add(n.getNodeValue());
 
            if(n.rightNode() != null){
                stack.push(n.rightNode());
            }
            if(n.leftNode() != null){
                stack.push(n.leftNode());
            }
 
        }
        return returnList;
    }
}