验证二叉树是否是查找树，可以通过查看它的中序遍历是否是升序的。

下面是AC代码：

1 /** 2      * Given a binary tree, determine if it is a valid binary search tree (BST). 3      * solution : 4      * inorder sequence is an ascending sequence! 5      * @param root 6      * @return 7      */ 8     public boolean isValidBST(TreeNode root){ 9         if(root == null)10             return true;11         LinkedList
     
       stack = new LinkedList
      
       ();12         //stack.push(root);13         14         TreeNode p = root;15         TreeNode pre = null;16         while(p!=null||!stack.isEmpty()){17             if(p!=null){18                 stack.push(p);19                 p = p.left;20             }else{21                 p = stack.pop();22                 if(pre!=null && pre.val>=p.val)23                     return false;24                 pre = p;25                 p = p.right;26             }27         }28         return true;29     }