Adding Undirected Edges

When the input says there is an undirected edge between $u$ and $v$, you must record it in both adjacency lists. I add $v$ to adj[u] and $u$ to adj[v].

adj[u].push_back(v)
adj[v].push_back(u)

If you only add one direction, a traversal from the other endpoint will miss the connection and you will report the wrong answer. Adding both entries takes $O(1)$ time and $O(1)$ extra space per edge, so you should always do it.