class Solution {
public:
    bool dfs(TreeNode* root,long long lower,long long uper)
    {
        if(root == nullptr)
        return true;

        if(root->val <= lower || root->val >= uper)
        return false;

        return dfs(root->left,lower,root->val) && dfs(root->right,root->val,uper);
    }
    bool isValidBST(TreeNode* root) {
        return dfs(root,LONG_MIN,LONG_MAX);
    }
};