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(When is an edge a bridge?)
For each tree edge where is the parent of in the DFS tree: Check if low[v] > disc[u]. If yes, is a bridge. Why? low[v] > disc[u] means the subtree of cannot reach or any ancestor of .
The edge is the only connection between the subtree and the rest of the graph. If low[v] <= disc[u], then there is a back edge providing an alternate path, so is not a bridge.
