class Solution {
public:
    void lur(vector<int>& ret,TreeNode* root)
    {
        if(root == nullptr) return ;
        lur(ret,root->left);
        ret.push_back(root->val);
        lur(ret,root->right);
    }
    vector<int> inorderTraversal(TreeNode* root) {
        // write code here
        vector<int> ret;
        lur(ret,root);
        return ret;
    }
};