Interview Question for Software Development Engineering Intern at Amazon.com:

using recursion

public static void preorderTraversal(Tree T,Node root){
      if(root==null){
    return "empty tree";
}
else{
    visit(T,root);
preorderTraversal(T,T,leftChild(root));
preorderTraversal(T,T,rightChild(root));
}
}

public static void preorderTraversal(TreeNode node) {
    Stack<TreeNode> s = new Stack<TreeNode>();
    while (node != null || !s.isEmpty()) {
        while (node != null) {
             visit(node);
             node = node.left;
        }
        if (!s.isEmpty()) {
             node = s.pop();
             node = node.right;
        }
    }
}