G面经prepare: Friends Recommendation

想想如果你用linkedin或者facebook， 给你一个人和他的朋友关系网，你会怎么给一个人推荐朋友

一个例子就是A-B, A-C, B - D, B - E, C - D，这个时候问我应该推荐谁给A，我说D,因为他是BC的共同好友，而E只是B的好友,到这我才明白干啥，就是给一个图和里面的一个节点A,用bfs从A出发，找出第二层中indegree度数最大节点

用HashMap<Character, HashSet<Character>>来建图

用HashMap<Character, Integer> SndIndegree来表示第二层indegree数目

用maxIndegree记录第二层Indegree最大值

用res记录第二层Indegree最大者

BFS

 package FriendRecommendation;
 import java.util.*;

 public class Solution {
     public char recommend(char tar, char[][] arr) {
         HashMap<Character, HashSet<Character>> graph = new HashMap<Character, HashSet<Character>>();
         HashMap<Character, Integer> SndIndegree = new HashMap<Character, Integer>();

         //build graph
         for (char[] edge : arr) {
             if (!graph.containsKey(edge[0])) graph.put(edge[0], new HashSet<Character>());
             if (!graph.containsKey(edge[1])) graph.put(edge[1], new HashSet<Character>());
             graph.get(edge[0]).add(edge[1]);
             if (!SndIndegree.containsKey(edge[0])) SndIndegree.put(edge[0], 0);
             if (!SndIndegree.containsKey(edge[1])) SndIndegree.put(edge[1], 0);
         }

         Queue<Character> queue = new LinkedList<Character>();
         HashSet<Character> visited = new HashSet<Character>();
         int level = 0;
         queue.offer(tar);
         visited.add(tar);
         int PNum = 1;
         int CNum = 0;
         int maxIndegree = 0;
         char res = '\0';
         while (!queue.isEmpty()) {
             char cur = queue.poll();
             PNum--;
             for (Character neigh : graph.get(cur)) {
                 if (level+1 == 2) {
                     if (neigh == tar) continue;
                     int curIndegree = SndIndegree.get(neigh)+1;
                     if (curIndegree > maxIndegree) res = neigh.charValue();
                     SndIndegree.put(neigh, curIndegree);
                 }
                 else { //not second level
                     if (!visited.contains(neigh)) {
                         queue.offer(neigh);
                         CNum++;
                         visited.add(neigh);
                     }
                 }
             }
             if (PNum == 0) {
                 PNum = CNum;
                 CNum = 0;
                 level++;
             }
         }
         return res;
     }

     /**
      * @param args
      */
     public static void main(String[] args) {
         // TODO Auto-generated method stub
         Solution sol = new Solution();
         char res = sol.recommend('A', new char[][]{{'A','B'},{'A','C'},{'B','D'},{'B','E'},{'C','D'},{'B','A'},{'C','A'}});
         System.out.println(res);
     }

 }