You are cloning the tree data every time you call dfs() function.

Try something like this:

void dfs(vector<vector<ll>>& tree, ll v)

A small note for you:

These type of questions can be asked in the tutorial blog of that contest. Creating a new blog to ask about simple problems with your solution is disturbing for the community as all the posts appear in the homepage as long as it's active.

Don't take it personally. Thanks.