Leetcode 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.

题目要我们找到一个graph 里 互相连接的component 的个数

1、使用 adjacent list 来表示一个图

2、遍历每个 节点 从1到N

3、DFS (深度优先搜索)， 用一个boolean array 来记录访问的节点

public class Solution {    public int countComponents(int n, int[][] edges) {        if (n <= 1) return n;                List<List<Integer>> adj_list = new ArrayList<>();                for (int i = 0; i < n; i++) {            adj_list.add(new ArrayList<Integer>());        }                for (int[] edge : edges) {            adj_list.get(edge[0]).add(edge[1]);            adj_list.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, adj_list);            }        }                return count;    }        private void dfs(boolean[] visited, int index, List<List<Integer>> adj_list) {        visited[index] = true;                for (int i : adj_list.get(index)) {            if (!visited[i]) dfs(visited, i, adj_list);        }    }}