题目：

题解：

type BSTIterator struct {
    stack []*TreeNode
    cur   *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.Val
    it.cur = it.cur.Right
    return val
}

func (it *BSTIterator) HasNext() bool {
    return it.cur != nil || len(it.stack) > 0
}