13 Strongly Connected Component.cpp

/**

Strongly Connected Component

**/

/** Which of the favors of your Lord will you deny ? **/

#include<bits/stdc++.h>
using namespace std;

#define LL long long
#define PII pair<int,int>
#define PLL pair<LL,LL>
#define MP make_pair
#define F first
#define S second
#define INF INT_MAX

#define ALL(x) (x).begin(), (x).end()
#define DBG(x) cerr << __LINE__ << " says: " << #x << " = " << (x) << endl

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

template<class TIn>
using indexed_set = tree<
                    TIn, null_type, less<TIn>,
                    rb_tree_tag, tree_order_statistics_node_update>;

/*
PBDS
-------------------------------------------------
1) insert(value)
2) erase(value)
3) order_of_key(value) // 0 based indexing
4) *find_by_order(position) // 0 based indexing

*/

inline void optimizeIO()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
}

const int nmax = 1e3+7;
const LL LINF = 1e17;

string to_str(LL x)
{
    stringstream ss;
    ss<<x;
    return ss.str();
}

//bool cmp(const PII &A,const PII &B)
//{
//
//}

int scc_num;

vector<int>graph[nmax];
vector<int>rev_graph[nmax];

int SCCMap[nmax];

stack<int>stk;

bool visited[nmax];

bool flag=true;

void forward_dfs(int node)
{
    visited[node]=true;

    for(int next:graph[node])
    {
        if(!visited[next])
            forward_dfs(next);
    }

    stk.push(node);
}

void reverse_dfs(int node)
{
    visited[node]=true;
    SCCMap[node]=scc_num;

    for(int next:rev_graph[node])
    {
        if(!visited[next])
            reverse_dfs(next);
    }
}

void init()
{
    for(int i=0; i<nmax; i++)
    {
        graph[i].clear();
        rev_graph[i].clear();
    }

    fill(visited,visited+nmax,false);

    while(!stk.empty())
        stk.pop();
}

int main()
{
    //freopen("out.txt","w",stdout);

    init();

    int nodes,edges;
    scanf("%d %d",&nodes,&edges);

    for(int j=1; j<=edges; j++)
    {
        int a,b;
        scanf("%d %d",&a,&b);

        graph[a].push_back(b); /** Directed Acyclic Graph (DAG) **/
        rev_graph[b].push_back(a);
    }

    //cout<<"Serial "<<serial<<endl;

    for(int j=1; j<=nodes; j++) /** 1 based indexing **/
        if(!visited[j])
            forward_dfs(j);

    fill(visited,visited+nmax,false);

    scc_num=1;

    while(!stk.empty())
    {
        int now=stk.top();
        stk.pop();

        if(!visited[now])
        {
            reverse_dfs(now);
            scc_num++;
        }
    }

    scc_num--;

    for(int i=1;i<=scc_num;i++)
    {
        cout<<"SCC : "<<i<<endl;

        for(int j=1;j<=nodes;j++)
            if(SCCMap[j]==i) cout<<j<<" ";
        cout<<endl;
    }


    return 0;
}