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$ curl repovive.com/roadmaps/graph-theory/graph-fundamentals/adding-undirected-edges

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$ curl repovive.com/roadmaps/graph-theory/graph-fundamentals/adding-undirected-edges

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Adding Undirected Edges - Graph Fundamentals | Graph Theory | Repovive

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Graph Theory37 sections · 1633 units81h total

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Adding Undirected Edges

Code Snippet

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.