题目：

题解：

type BSTIterator struct {stack []*TreeNodecur   *TreeNode
}func Constructor(root *TreeNode) BSTIterator {return BSTIterator{cur: root}
}func (it *BSTIterator) Next() int {for node := it.cur; node != nil; node = node.Left {it.stack = append(it.stack, node)}it.cur, it.stack = it.stack[len(it.stack)-1], it.stack[:len(it.stack)-1]val := it.cur.Valit.cur = it.cur.Rightreturn val
}func (it *BSTIterator) HasNext() bool {return it.cur != nil || len(it.stack) > 0
}