173. Binary Search Tree Iterator

建一个stack，最末的元素始终是最小的那个。

所以对于初始化，存到最左下角的那个节点。

对于找到next，就是存到，当前节点的右子节点的最左下角（如果有的话）。

public class BSTIterator {
    Stack<TreeNode> stack;
    
    public BSTIterator(TreeNode root) {
        stack = new Stack<TreeNode>();
        TreeNode cur = root;
        while(cur != null) {
            stack.push(cur);
            cur = cur.left;
        }
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode cur = stack.pop();
        int next = cur.val;
        cur = cur.right;
        while(cur != null) {
            stack.push(cur);
            cur = cur.left;
        }
        return next;
    }
}