BT_Inorder_Rec.cpp

class Solution {
private:
    void dfs(vector<int>& res, TreeNode* root){
        if(root == nullptr) return;
        dfs(res, root->left);
        res.push_back(root->val);
        dfs(res, root->right);
    }
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        dfs(res, root);
        return res;
    }
};