articulation_points.py

# Finding Articulation Points in Undirected Graph
def computeAP(l):
    n = len(l)
    outEdgeCount = 0
    low = [0] * n
    visited = [False] * n
    isArt = [False] * n

    def dfs(root, at, parent, outEdgeCount):
        if parent == root:
            outEdgeCount += 1
        visited[at] = True
        low[at] = at

        for to in l[at]:
            if to == parent:
                pass
            elif not visited[to]:
                outEdgeCount = dfs(root, to, at, outEdgeCount)
                low[at] = min(low[at], low[to])

                # AP found via bridge
                if at < low[to]:
                    isArt[at] = True
                # AP found via cycle
                if at == low[to]:
                    isArt[at] = True
            else:
                low[at] = min(low[at], to)
        return outEdgeCount

    for i in range(n):
        if not visited[i]:
            outEdgeCount = 0
            outEdgeCount = dfs(i, i, -1, outEdgeCount)
            isArt[i] = (outEdgeCount > 1)

    for x in range(len(isArt)):
        if isArt[x] == True:
            print(x)

# Adjacency list of graph
l = {0:[1,2], 1:[0,2], 2:[0,1,3,5], 3:[2,4], 4:[3], 5:[2,6,8], 6:[5,7], 7:[6,8], 8:[5,7]}
computeAP(l)