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);
    }

}