题目：

题解：

func isValidBST(root *TreeNode) bool {
    stack := []*TreeNode{}
    inorder := math.MinInt64
    for len(stack) > 0 || root != nil {
        for root != nil {
            stack = append(stack, root)
            root = root.Left
        }
        root = stack[len(stack)-1]
        stack = stack[:len(stack)-1]
        if root.Val <= inorder {
            return false
        }
        inorder = root.Val
        root = root.Right
    }
    return true
}