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
1 package FriendRecommendation; 2 import java.util.*; 3 4 public class Solution { 5 public char recommend(char tar, char[][] arr) { 6 HashMap<Character, HashSet<Character>> graph = new HashMap<Character, HashSet<Character>>(); 7 HashMap<Character, Integer> SndIndegree = new HashMap<Character, Integer>(); 8 9 10 //build graph 11 for (char[] edge : arr) { 12 if (!graph.containsKey(edge[0])) graph.put(edge[0], new HashSet<Character>()); 13 if (!graph.containsKey(edge[1])) graph.put(edge[1], new HashSet<Character>()); 14 graph.get(edge[0]).add(edge[1]); 15 if (!SndIndegree.containsKey(edge[0])) SndIndegree.put(edge[0], 0); 16 if (!SndIndegree.containsKey(edge[1])) SndIndegree.put(edge[1], 0); 17 } 18 19 Queue<Character> queue = new LinkedList<Character>(); 20 HashSet<Character> visited = new HashSet<Character>(); 21 int level = 0; 22 queue.offer(tar); 23 visited.add(tar); 24 int PNum = 1; 25 int CNum = 0; 26 int maxIndegree = 0; 27 char res = ' '; 28 while (!queue.isEmpty()) { 29 char cur = queue.poll(); 30 PNum--; 31 for (Character neigh : graph.get(cur)) { 32 if (level+1 == 2) { 33 if (neigh == tar) continue; 34 int curIndegree = SndIndegree.get(neigh)+1; 35 if (curIndegree > maxIndegree) res = neigh.charValue(); 36 SndIndegree.put(neigh, curIndegree); 37 } 38 else { //not second level 39 if (!visited.contains(neigh)) { 40 queue.offer(neigh); 41 CNum++; 42 visited.add(neigh); 43 } 44 } 45 } 46 if (PNum == 0) { 47 PNum = CNum; 48 CNum = 0; 49 level++; 50 } 51 } 52 return res; 53 } 54 55 56 /** 57 * @param args 58 */ 59 public static void main(String[] args) { 60 // TODO Auto-generated method stub 61 Solution sol = new Solution(); 62 char res = sol.recommend('A', new char[][]{{'A','B'},{'A','C'},{'B','D'},{'B','E'},{'C','D'},{'B','A'},{'C','A'}}); 63 System.out.println(res); 64 } 65 66 }