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(Child cannot bypass parent)
For a non-root vertex with child in the DFS tree: If low[v] >= disc[u], then is an articulation point. Why? low[v] >= disc[u] means the subtree rooted at cannot reach any vertex discovered before .
The subtree can reach (when low[v] = disc[u]) or only later vertices (when low[v] > disc[u]). Either way, is the gateway. Remove , and the subtree disconnects.
