How do I actually delete the node(s)?

Question

I am solving a question on LeetCode.com:

Given the root of a binary tree, collect a tree's nodes as if you were doing this:
a. Collect all the leaf nodes.
b. Remove all the leaf nodes.
c. Repeat until the tree is empty.

                                    

For the input root = [1,2,3,4,5] (image above), the output should be: [[4,5,3],[2],[1]].

This comes down to finding the height of each node of the tree:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> m;
    
    int dfs(TreeNode* root) {
        if(!root) return -1;
        
        int l=dfs(root->left);
        int r=dfs(root->right);
        int height=max(l,r)+1;
        
        if(height==m.size()) m.push_back({});
        
        m[height].push_back(root->val);
        
        //delete root;
        
        return height;
    }

    vector<vector<int>> findLeaves(TreeNode* root) {
        dfs(root);
        
        return m;
    }
};

The above code gets accepted, but that is because the OJ does not actually check if the nodes were deleted or not. My question is, how do I delete the nodes? Specifically:

a. If I add delete (commented line above), I get a runtime error;
b. I cannot just set root=nullptr like the Java folks, since C++ does not have garbage collection by default and so the root node would not really be deleted (the memory would continue to be occupied).
c. I don't think we can delete the root node anywhere else.

So how do I go about actually deleting the node?

Thanks!