題目原文：

Given a binary tree where each ???????? contains a key, determine whether it is a binary search tree. Use extra space proportional to the height of the tree.

分析：

就是遞歸遍歷BST，將每個節點x分別與x.left和x.right比大小。

 1 public boolean isBST(BST<Key,Value> bst){
 2     return isBST(root);
 3 }
 4 private
 
 boolean isBST(Node x){
 5     if(x.left.key.compareTo(x.key) != -1) return false;
 6     if(x.key.compareTo(x.right.key)!= -1) return false;
 7     if(x.left != null ) return isBST(x.left);
 8     if(x.right != null ) return isBST(x.right);
 9     return true;
10 }

Coursera Algorithms week4 基礎標簽表 練習測驗：Check if a binary tree is a BST