今日头条笔试第一题

思路：简单的集合合并

合并策略：初始自己是一个集合，集合个数为n；

遍历第i名同学————第n名同学，i同学和他朋友们依次判断是否是一个集合，如果没在一个集合，进行合并，n--；

数据结构：并查集结构

import java.util.*;

public class Main {

public static class Node {

public Set friends = new HashSet<>();

}

public static HashMap fatherMap;

public static HashMap sizeMap;

public static void main(String[] args) {

//1、输入数据，组织数据结构

Scanner sc = new Scanner(System.in);

int n = sc.nextInt();

if (n <= 0 || n >= 100000) {

System.out.println(0);

return;

}

Node[] nodes = new Node[n + 1];

int tmp;

fatherMap = new HashMap();

sizeMap = new HashMap();

for (int i = 1; i <= n; ++i) {

nodes[i] = new Node();

fatherMap.put(nodes[i], nodes[i]);

sizeMap.put(nodes[i], 1);

while ((tmp = sc.nextInt()) != 0) {

nodes[i].friends.add(tmp);

}

}

//2、遍历每个节点，根据合并策略进行合并

for (int i = 1; i < nodes.length; i++) {

if (nodes[i].friends.size() == 0) continue;

for (Iterator iterator = nodes[i].friends.iterator(); iterator.hasNext(); ) {

Integer one = iterator.next();

if (!isSameSet(nodes[i], nodes[one])) {

union(nodes[i], nodes[one]);

n--;

}

}

}

//3、result

System.out.println(n);

}

private static Node findHead(Node node) {

Node father = fatherMap.get(node);

if (father != node) {

father = findHead(father);

}

fatherMap.put(node, father);

return father;

}

public static boolean isSameSet(Node a, Node b) {

return findHead(a) == findHead(b);

}

public static void union(Node a, Node b) {

if (a == null || b == null) {

return;

}

Node aHead = findHead(a);

Node bHead = findHead(b);

if (aHead != bHead) {

int aSetSize = sizeMap.get(aHead);

int bSetSize = sizeMap.get(bHead);

if (aSetSize <= bSetSize) {

fatherMap.put(aHead, bHead);

sizeMap.put(bHead, aSetSize + bSetSize);

} else {

fatherMap.put(bHead, aHead);

sizeMap.put(aHead, aSetSize + bSetSize);

}

}

}

}