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Number of Connected Components in an Undirected Graph

323. Number of Connected Components in an Undirected Graph

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

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

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

        boolean[] visited = new boolean[n];
        int count = 0;

        for (int i = 0; i < n; i++) {
            if (!visited[i]) {
                count++;
                dfs(visited, i, graph);
            }
        }

        return count;
    }

    void dfs(boolean[] visited, int index, List<List<Integer>> graph) {
        visited[index] = true;

        for (int i : graph.get(index)) {
            if (!visited[i]) {
                dfs(visited, i, graph);
            }
        }
    }
}

Hope this helps,
Michael