i think you need to understand how the code above works instead of just taking it its basically creating an array of vectors every slot of the array represent a node so lets say i want to add a link from node u to node v then i will just push v in the uth slot thats why i need to do something like this: e[u].push_back({v, whatever_weight}) now, we want to make undirected graph just instead just adding u to slot v we also add v to slot u so conclusion just add e[v].push_back({u, w}) as well to make an undirected graph