计算朋友圈

从每个未访问过的节点(用户)出发，进行 dfs 搜索。

dfs 的次数，就是朋友圈的数目。

import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;
public class Main {
  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    int n = scanner.nextInt();
    String[][] relations = new String[n][n];
    for (int i = 0; i < n; i++) {
      relations[i] = scanner.next().split(",");
    }
    scanner.close();
    System.out.println(friendCircle(relations));
  }
  private static int friendCircle(String[][] relations) {
    int n = relations.length;
    // graph[i] -> 用户 i 的所有朋友
    Set[] graph = new HashSet[n];
    for (int i = 0; i < n; i++) {
      graph[i] = new HashSet();
      for (int j = 0; j < n; j++) {
        int relation = Integer.parseInt(relations[i][j]);
        if (relation == 1) {
          graph[i].add(j);
        }
      }
    }
    int ans = 0;
    boolean[] visited = new boolean[n];
    for (int i = 0; i < n; i++) {
      if (!visited[i]) {
        ans++;
        dfs(graph, i, visited);
      }
    }
    return ans;
  }
  private static void dfs(Set[] graph, int cur, boolean[] visited) {
    visited[cur] = true;
    for (int next : graph[cur]) {
      if (!visited[next]) {
        dfs(graph, next, visited);
      }
    }
  }
}