No.98 Validate Binary Search Tree

//Given a binary tree, determine if it is a valid binary search tree (BST).

//solution

  bool isValidBST(TreeNode* root) 
    {
        if(root==NULL)
        return true;
        vector<int> res=inorder(root);
        for(int i=0;i<res.size()-1;i++)
            if(res[i]>=res[i+1])
            return false;
        return true; 
    }

    vector<int> inorder(TreeNode* root)
    {
        vector<int> res;
        vector<int> left;
        vector<int> right;
        if(root==NULL)
        return res;
        left=inorder(root->left);
        right=inorder(root->right);
        left.push_back(root->val);
        left.insert(left.end(),right.begin(),right.end());
        res=left;
        return res;

    }