(SPOJ) Rodovia - Solution

Link to the problem: http://br.spoj.com/problems/RODOVI13/

The solution below used Kosaraju's algorithm to solve this problem.


import java.io.*;
import java.util.*;

class Main {
    public HashMap<Integer, ArrayList<Integer>> adjList;
    public HashMap<Integer, ArrayList<Integer>> adjListReverse;
    public HashSet<Integer> visited;
    public Deque<Integer> stack;
  
    public void dfsReverse(int node) {
        if (visited.contains(node)) {
            return;
        }
        visited.add(node);
      
        ArrayList<Integer> reachNodes = adjListReverse.get(node);
        for (int i = 0; i < reachNodes.size(); i++) {
            node = reachNodes.get(i);
            dfsReverse(node);
        }
    }
  
    public void dfs(int node) {
        if (visited.contains(node)) {
            return;
        }
        visited.add(node);
       
        int oldNode = node;      
        ArrayList<Integer> reachNodes = adjList.get(node);
        for (int i = 0; i < reachNodes.size(); i++) {
            node = reachNodes.get(i);
            dfs(node);
        }
        stack.push(oldNode);
    }
  
    public void process() throws NumberFormatException, IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
      
        String line = br.readLine();
        int numCities = Integer.parseInt(line);
      
        adjList = new HashMap<>();
        adjListReverse = new HashMap<>();
        for (int i = 1; i <= numCities; i++) {
            adjList.put(i, new ArrayList<Integer>());
            adjListReverse.put(i, new ArrayList<Integer>());
        }
      
        for (int i = 0; i < numCities; i++) {
            line = br.readLine();
            String[] s = line.split("\\s");
            int c1 = Integer.parseInt(s[0]);
            int c2 = Integer.parseInt(s[1]);
          
            adjList.get(c1).add(c2);
            adjListReverse.get(c2).add(c1);
        }
      
        stack = new ArrayDeque();
        visited = new HashSet<>();
        for (int i = 1; i <= numCities; i++) {
            if (visited.contains(i)) {
                continue;
            }
            dfs(i);
        }
      
        int count = 0;
        int size = stack.size();
        visited = new HashSet<>();
        for (int i = 0; i < size; i++) {
            int elem = stack.pop();
            if (visited.contains(elem)) {
                continue;
            }
            dfsReverse(elem);
            count++;
        }
      
        if (count == 1) {
            bw.write("S\n");
        }
        else {
            bw.write("N\n");
        }
          
        bw.flush();
        bw.close();
      
        return;
    }
  
    public static void main(String[] args) throws NumberFormatException, IOException {
        Main m = new Main();
        m.process();
      
        System.exit(0);
    }
}