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https://drive.google.com/drive/u/0/folders/1rHeA70oqU5kvGD5APwFRrMouMqgOy18s

 
class Solution {
public:
   bool ceck(TreeNode* root,long lmax,long rmin){
       if(root==NULL) return true;
       //if(root->left==NULL && root->right==NULL) return true;

       
    
        bool l=ceck(root->left,lmax,root->val); // lmax < root->val << rmin
       
       
       if(root->val <= lmax) return false;
       if(root->val >= rmin) return false;

       
       bool r=ceck(root->right,root->val,rmin);
       
       return l&r;
   }

    bool isValidBST(TreeNode* root) {
       
        bool res =ceck(root,LONG_MIN,LONG_MAX);
        return res;
    }
};