2Sigma OA prepare: Friends Circle

DFS & BFS:

关键在于构造graph

 package twoSigma;

 import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.LinkedList;
 import java.util.Queue;

 public class FriendCircle1 {
     ArrayList<ArrayList<Integer>> graph;
     public int friendCircles(String[] friends) {
         graph = new ArrayList<ArrayList<Integer>>();
         for (int i=0; i<friends.length; i++) {
             ArrayList<Integer> item = new ArrayList<Integer>();
             for (int j=0; j<friends[0].length(); j++) {
                 if (i!=j && friends[i].charAt(j) == 'Y') {
                     item.add(j);
                 }
             }
             graph.add(new ArrayList<Integer>(item));
         }
         HashSet<Integer> visited = new HashSet<Integer>();
         int res = 0;
         for (int i=0; i<graph.size(); i++) {
             if (!visited.contains(i)) {
                 res++;
                 //bfs(i, visited);
                 dfs(i, visited);
             }
         }
         return res;
     }

     public void bfs(int i, HashSet<Integer> visited) {
         visited.add(i);
         Queue<Integer> q = new LinkedList<Integer>();
         q.offer(i);
         while (!q.isEmpty()) {
             int cur = q.poll();
             ArrayList<Integer> curFriends = graph.get(cur);
             for (int each : curFriends) {
                 if (!visited.contains(each)) {
                     visited.add(each);
                     q.offer(each);
                 }
             }
         }
     }

     public void dfs(int i, HashSet<Integer> visited) {
         if (visited.contains(i)) return;
         visited.add(i);
         ArrayList<Integer> friends = graph.get(i);
         for (int friend : friends) {
             dfs(friend, visited);
         }
     }

     /**
      * @param args
      */
     public static void main(String[] args) {
         // TODO Auto-generated method stub
         FriendCircle1 sol = new FriendCircle1();
         //String[] input = new String[]{"YNYNNN", "NYNYNN", "YNYNNN", "NYNYNN", "NNNNYY", "NNNNYY"};
         //String[] input = new String[]{"YYNN", "YYYN", "NYYN", "NNNY"};
         //String[] input = new String[]{"YNN", "NYN", "NNY"};
         String[] input = new String[]{"YYY", "YYY", "YYY"};
         int count = sol.friendCircles(input);
         System.out.println(count);
     }

 }

如果想把String[] array转化为2d char array， code如下，写的时候这里出了点小问题。friends是String[]

         char[][] arr = new char[friends.length][];
         for (int i=0; i<friends.length; i++) {
             arr[i] = friends[i].toCharArray();
         }

Union Find

 package twoSigma;

 public class FriendCircle2 {
     public int friendCircles(String[] friends) {
         Unionfind uf = new Unionfind(friends.length);
         for (int i=0; i<friends.length; i++) {
             for (int j=0; j<friends[i].length(); j++) {
                 if (i != j && friends[i].charAt(j)=='Y')
                     uf.union(i, j);
             }
         }
         return uf.count;
     }

     public class Unionfind {
         int[] ids;
         int count;
         public Unionfind(int num) {
             this.ids = new int[num];
             this.count = num;
             for (int i=0; i<num; i++) {
                 ids[i] = i;
             }
         }

         public int findId(int i) {
             return ids[i];
         }

         public boolean isConnected(int i1, int i2) {
             return findId(i1)==findId(i2);
         }

         public boolean union(int i1, int i2) {
             if (!isConnected(i1, i2)) {
                 for (int i=0; i<ids.length; i++) {
                     if (ids[i] == findId(i2)) {
                         ids[i] = findId(i1);
                     }
                 }
                 count--;
                 return true;
             }
             return false;
         }
     }

     /**
      * @param args
      */
     public static void main(String[] args) {
         // TODO Auto-generated method stub
         FriendCircle2 sol = new FriendCircle2();
         String[] input = new String[]{"YNYNNN", "NYNYNN", "YNYNNN", "NYNYNN", "NNNNYY", "NNNNYY"};
         //String[] input = new String[]{"YYNN", "YYYN", "NYYN", "NNNY"};
         //String[] input = new String[]{"YNN", "NYN", "NNY"};
         //String[] input = new String[]{"YYY", "YYY", "YYY"};
         int count = sol.friendCircles(input);
         System.out.println(count);
     }

 }