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# Program to find out the critical and pseudo-critical edges in a graph in Python

Suppose, we are given a graph that contains n vertices numbered 0 to n -1. The graph is undirected and each edge has a weight. So given the graph, we have to find out the critical and the pseudo-critical edges in the graphs MST. An edge is called a critical edge if deletion of that edge causes the MST weight to increase. A pseudo-critical edge is an edge that can appear in all the graphs MSTs, but not all. We find out the index of the edges given the graph as input.

So, if the input is like

and number of vertices is 5, then the output will be [[], [0, 1, 2, 3, 4]] There are no critical edges in the given graph and all the edges are pseudo-critical. As all the edges have the same weight, any 3 edges from the graph will make an MST.

To solve this, we will follow these steps −

Define a function find_mst() . This will take num_vertices, graph, init := null, exl := null

Define one helper function visit() . This will take u

k[u] := True

for each v, w in of graph[u, an empty list], do

if exl and u is in exl and v is in exl, then

go for next iteration

if not k[v] is True, then

push triplet (w,u,v) into heap tmp

res := 0

k := a new list of size num_arrays containing value False

tmp := a new heap

if init is non-null, then

u := init

v := init

w := init

res := res + w

k[u] := True

k[v] := True

visit(u) or visit(v)

otherwise,

visit(0)

while tmp is not empty, do

w := pop smallest item from heap tmp

u := pop smallest item from heap tmp

v := pop smallest item from heap tmp

if k[u] and k[v] is non-zero, then

go for next iteration

res := res + w

if not k[u] is True, then

visit(u)

if not k[v] is True, then

visit(v)

return res if all of k are True, otherwise return infinity

From the main method, do the following:

graph := given graph

temp := find_mst(num_vertices, graph)

c_edge := a new list

p_edge := a new list

for i in range 0 to size of edges, do

if find_mst(num_vertices, graph, exl = edges[i, index 2 to end]) > temp, then

insert i at the end of c_edge

otherwise, if find_mst(num_vertices, graph, init = edges[i]) is same as temp, then

insert i at the end of p_edge

return [c_edge, p_edge]

## Example

Let us see the following implementation to get better understanding

from heapq import heappop, heappush def solve(num_vertices, edges): graph = dict() for u, v, w in edges: graph.setdefault(u, []).append((v, w)) graph.setdefault(v, []).append((u, w)) temp = find_mst(num_vertices, graph) c_edge, p_edge = [], [] for i in range(len(edges)): if find_mst(num_vertices, graph, exl = edges[i][:2]) > temp: c_edge.append(i) elif find_mst(num_vertices, graph, init = edges[i]) == temp: p_edge.append(i) return [c_edge, p_edge] def find_mst(num_vertices, graph, init = None, exl = None): def visit(u): k[u] = True for v, w in graph.get(u, []): if exl and u in exl and v in exl: continue if not k[v]: heappush(tmp, (w, u, v)) res = 0 k = [False] * num_vertices tmp = [] if init: u, v, w = init res += w k[u] = k[v] = True visit(u) or visit(v) else: visit(0) while tmp: w, u, v = heappop(tmp) if k[u] and k[v]: continue res += w if not k[u]: visit(u) if not k[v]: visit(v) return res if all(k) else inf print(solve(5, [[0,1,10],[1,2,10],[2,3,10],[3,4,10],[4,0,10]]))

## Input

5, [[0,1,10],[1,2,10],[2,3,10],[3,4,10],[4,0,10]]

## Output

[[], [0, 1, 2, 3, 4]]

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