```
public class BSTIterator {
TreeNode root;
ArrayList<Integer> ls = new ArrayList<Integer>();
int point = 0;
int n = 0;
public BSTIterator(TreeNode root) {
this.root = root;
inorder(root);
n = ls.size();
}
private void inorder(TreeNode root) {
if(root == null) return;
if(root.left != null) inorder(root.left);
ls.add(root.val);
if(root.right != null) inorder(root.right);
}
/** @return whether we have a next smallest number */
public boolean hasNext() {
if(point < n) return true;
else return false;
}
/** @return the next smallest number */
public int next() {
return ls.get(point++);
}
```

}

- use inorder to traverse the tree and save in ArrayList when initiate iterator
- hasNext() means check whether point in the size of list

next() means move point in list

I guess moving point in list is quicker than pop() in stack.