Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

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.
A single node tree is a BST

1.需要两个值maxValue, minValue 来记录当前节点应该的取值范围。

`/** * Definition of TreeNode: * class TreeNode { * public: *     int val; *     TreeNode *left, *right; *     TreeNode(int val) { *         this->val = val; *         this->left = this->right = NULL; *     } * } */class Solution {public:    /**     * @param root: The root of binary tree.     * @return: True if the binary tree is BST, or false     */    bool helper(TreeNode *node,long minVal,long maxVal){        if(node==NULL){            return true;        }        if(node->val<=minVal || node->val>=maxVal){            return false;        }        return helper(node->left,minVal,node->val) & helper(node->right,node->val,maxVal);    }    bool isValidBST(TreeNode *root) {        // write your code here        return helper(root,LONG_MIN,LONG_MAX);    }};`

2.判断中序遍历是否有序

`这里写代码片/** * Definition of TreeNode: * class TreeNode { * public: *     int val; *     TreeNode *left, *right; *     TreeNode(int val) { *         this->val = val; *         this->left = this->right = NULL; *     } * } */class Solution {public:    /**     * @param root: The root of binary tree.     * @return: True if the binary tree is BST, or false     */    bool helper(TreeNode *root, long & pre){        if(root==NULL){            return true;        }        if(helper(root->left,pre)==false){            return false;        }        if(root->val<=pre){            return false;        }else{            pre=root->val;        }        if(helper(root->right,pre)==false){            return false;        }        return true;    }    bool isValidBST(TreeNode *root) {        // write your code here        long pre=LONG_MIN;        return helper(root,pre);    }};`