判断一棵树是否为二叉搜索树

A binary search tree (BST) is a node based binary tree data structure which has the following properties.

• The left subtree of a node contains only nodes with keys
  less than the node’s key.
• The right subtree of a node contains only nodes with keys
  greater than the node’s key.
• Both the left and right subtrees must also be binary search
  trees.
  对二叉树进行中序遍历，判断该序列是否为升序序列