Binary Tree Traversal

1. BST iterator (Inorder traversal)

   ref: https://leetcode.com/problems/binary-search-tree-iterator/

    public class BSTIterator {
    private Stack<TreeNode> stack;
    private TreeNode prev;
   
    public BSTIterator(TreeNode root) {
        prev = null;
        stack = new Stack<TreeNode>();
        while (root != null) {
            stack.push(root);
            root = root.left;
        }
    }
   
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty() || (prev != null && prev.right != null);
    }
   
    /** @return the next smallest number */
    public int next() {
   
        if (prev != null && prev.right != null) {
            prev = prev.right;
            while (prev != null) {
                stack.push(prev);
                prev = prev.left;
            }
   
        }
        prev = stack.pop();
        return prev.val;
    }
   }