使用 Tarjan 算法在有向图中查找桥的特例

Question

I am trying to get better understanding of Tarjan's algorithm for finding SCC, articulation points and bridges. I am considering a special case where the graph contains only 2 nodes with edges 0->1 and 1->0. The following code will output [0,1] as a bridge.

class Solution(object):
    def criticalConnections(self, n, connections):
        """
        :type n: int
        :type connections: List[List[int]]
        :rtype: List[List[int]]
        """
        g = defaultdict(set)
        pre = [-1]*n
        low = [-1]*n
        cnt = [0]
        for c in connections:
            g[c[0]].add(c[1]) # undirected graph, connect 
            g[c[1]].add(c[0]) # in both directions
        ans = []
        def dfs(edge):            
            v, w = edge
            pre[w] = cnt[0]
            low[w] = pre[w]
            cnt[0] += 1
            for i in g[w]:
                if i == v: continue # we don't want to go back through the same path.
                                    # if we go back is because we found another way back                   
                if pre[i] == -1:
                    dfs((w,i))          
                    # low[i] > pre[w] indicates no back edge to
                    # w's ancesters; otherwise, low[i] will be 
                    # < pre[w]+1 since back edge makes low[i] smaller           
                    if low[i] > pre[w]: 
                    #print(low[i], pre[w]+1, (w,i))                       
                        ans.append([w,i])                
                    low[w] = min(low[w], low[i]) # low[i] might be an ancestor of w
                else: # if i was already discovered means that we found an ancestor
                    low[w] = min(low[w], pre[i]) # finds the ancestor with the least 
                                                 # discovery time
               
                
        dfs((-1,0))
        
        return ans
print(Solution().criticalConnections(2, [[0,1],[1,0]]))

However, from many discussions online, after removing node 1, node 0 can still be considered as connected (to itself) which means edge 0->1 is not a bridge. Am I missing something here? Or Tarjan's algorithm is not suitable for this kind of degenerate graph with 2 nodes?

Answer 1

A bridge in a directed graph is an edge whose deletion increases the graph's number of strongly connected components, and the number connected components when the graph is undirected. So when you remove any edge in your graph then the number of strongly connected components increases so the output of this code is correct in this case.