NumberOfConnectedComponentsInAnUndirectedGraph.java

package com.fishercoder.solutions;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

/**
 *  Given n nodes labeled from 0 to n - 1 and a list of undirected edges (each edge is a pair of nodes), write a function to find the number of connected components in an undirected graph.

 Example 1:

 0          3
 |          |
 1 --- 2    4

 Given n = 5 and edges = [[0, 1], [1, 2], [3, 4]], return 2.

 Example 2:

 0           4
 |           |
 1 --- 2 --- 3

 Given n = 5 and edges = [[0, 1], [1, 2], [2, 3], [3, 4]], return 1.

 Note:
 You can assume that no duplicate edges will appear in edges. Since all edges are undirected, [0, 1] is the same as [1, 0] and thus will not appear together in edges.
 */
public class NumberOfConnectedComponentsInAnUndirectedGraph {

    public int countComponents(int n, int[][] edges) {
        if(n <= 1) return n;

        List<List<Integer>> adList = new ArrayList<List<Integer>>();
        for(int i = 0; i < n; i++){
            adList.add(new ArrayList<Integer>());
        }

        for(int[] edge : edges){
            adList.get(edge[0]).add(edge[1]);
            adList.get(edge[1]).add(edge[0]);
        }

        for(List<Integer> list : adList){
            for(int i : list){
                System.out.print(i + ", ");
            }
            System.out.println();
        }

        boolean[] visited = new boolean[n];
        int count = 0;
        for(int i = 0; i < n; i++){
            if(!visited[i]){
                count++;
                Queue<Integer> q = new LinkedList<Integer>();
                q.offer(i);
                while(!q.isEmpty()){
                    int index = q.poll();
                    visited[index] = true;
                    for(int j : adList.get(index)){
                        if(!visited[j]){
                            q.offer(j);
                        }
                    }
                }
            }
        }

        return count;
    }

}