Validate Binary Search Tree

Example 1:

    2
   /   1   3

Input: [2,1,3]
Output: true
Example 2:

    5
   /   1   4
     /     3   6

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

struct TreeNode 
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution
{
public:
    bool isValidBST(TreeNode* root)
    {
        if(!root)
            return true;
        
        return isValidTreeBSTCore(root,nullptr,nullptr);
    }
private:    
    bool isValidTreeBSTCore(TreeNode *root,TreeNode *left,TreeNode *right)
    {
        if(root==nullptr)        
            return true;
        
        if(left&&left->val>=root->val)
            return false;
        if(right&&right->val<=root->val)
            return false;
            
        if(isValidTreeBSTCore(root->left,left,root)&&isValidTreeBSTCore(root->right,root,right))
            return true;
            
        return false;
    }
};