算法描述：

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.

Example 1:

Input:
    2
   /   1   3
Output: true

Example 2:

    5
   /   1   4
     /     3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node‘s value
             is 5 but its right child‘s value is 4.

解題思路：遞歸求解，二叉排序樹左邊所有值小於根節點，右邊所有值大於根節點。

    bool isValidBST(TreeNode* root) {
        
 
if(root==nullptr) return true;
        return helper(root, nullptr,nullptr);
    }
    
    bool helper(TreeNode* root, TreeNode* minModel, TreeNode* maxModel){
        if(root==nullptr) return true;
        if(minModel !=nullptr && root->val <= minModel->val) return false;
        
 
if(maxModel !=nullptr && root->val >= maxModel->val) return false;
        return helper(root->left, minModel, root) && helper(root->right, root, maxModel);
    }

LeetCode-98-Validate Binary Search Tree