Discuss

#include <iostream>

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/


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


class Solution
{
private:
    bool isValidBST(TreeNode* root, int &MinValue, int &MaxValue)
    {
        if (!root)
        {
            return true;
        }


        if (root->left)
        {
            if (root->val <= root->left->val)
            {
                return false;
            }

            int LeftMinValue = 0;
            int LeftMaxValue = 0;
            if (!isValidBST(root->left, LeftMinValue, LeftMaxValue))
            {
                return false;
            }
            else
            {
                MinValue = LeftMinValue;
                if (LeftMaxValue != LeftMinValue)
                {
                    if (root->val <= LeftMaxValue)
                    {
                        return false;
                    }
                }
            }
        }
        else
        {
            MinValue = root->val;
        }


        if (root->right)
        {
            if (root->val >= root->right->val)
            {
                return false;
            }

            int RightMinValue = 0;
            int RightMaxValue = 0;

            if (!isValidBST(root->right, RightMinValue, RightMaxValue))
            {
                return false;
            }
            else
            {
                MaxValue = RightMaxValue;
                if (RightMaxValue != RightMinValue)
                {
                    if (root->val >= RightMinValue)
                    {
                        return false;
                    }
                }
            }
        }
        else
        {
            MaxValue = root->val;
        }

        return true;
    }

public:
    bool isValidBST(TreeNode* root)
    {
        int MinValue = 0;
        int MaxValue = 0;
        bool IsLeaf = true;

        return isValidBST(root, MinValue, MaxValue);
    }
};