1107 Social Clusters （30 分）
When register on a social network, you are always asked to specify your hobbies in order to find some potential friends with the same hobbies. A social cluster is a set of people who have some of their hobbies in common. You are supposed to find all the clusters.
Input Specification:
Each input file contains one test case. For each test case, the first line contains a positive integer N (≤1000), the total number of people in a social network. Hence the people are numbered from 1 to N. Then N lines follow, each gives the hobby list of a person in the format:
K?i??: h?i??[1] h?i??[2] ... h?i??[K?i??]
where K?i?? (>0) is the number of hobbies, and h?i??[j] is the index of the j-th hobby, which is an integer in [1, 1000].
Output Specification:
For each case, print in one line the total number of clusters in the network. Then in the second line, print the numbers of people in the clusters in non-increasing order. The numbers must be separated by exactly one space, and there must be no extra space at the end of the line.
Sample Input:
8
3: 2 7 10
1: 4
2: 5 3
1: 4
1: 3
1: 4
4: 6 8 1 5
1: 4
Sample Output:
3
4 3 1这道题考察并查集。
开始遇到了数组越界的问题，后来调试发现是最近考研复习计算机系统基础遇到的一个知识点：
1 for (i = 1; i < 1001; i++)
2 {
3 for (j = 0; j < hobby[i].size() - 1; j++)
4 _union(hobby[i][j], hobby[i][j + 1]);
5 }
我开始是这么写的，看似不会发生数组越界的问题，但是因为hobby[i].size()是无符号数unsigned型，当hobby[i].size()等于0时，减去1就成了无符号数2^32-1，所以发生了越界。
看来了解底层的一些原理确实很重要，usigned型要慎用。
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
vector<vector<int>> hobby(1001);
vector<int> father(1001, -1);
void _union(int i, int j);
int find(int i);
int main()
{
int N;
cin >> N;
int i, j, k, t;
char c;
for (i = 1; i <= N; i++)
{
cin >> k >> c;
for (j = 0; j < k; j++)
{
cin >> t;
hobby[t].push_back(i);
}
}
for (i = 1; i < 1001; i++)
{
for (j = 1; j < hobby[i].size(); j++)
_union(hobby[i][j], hobby[i][j - 1]);
}
vector<int> cluster;
for (i = 1; i <= N; i++)
{
if (father[i] < 0)
cluster.push_back(-father[i]);
}
sort(cluster.begin(), cluster.end());
cout << cluster.size() << endl << cluster[cluster.size() - 1];
for (i = cluster.size() - 2; i >= 0; i--) cout << " " << cluster[i];
return 0;
}
void _union(int i, int j)
{
int a = find(i);
int b = find(j);
if (a == b) return;
if (father[a] < father[b])
{
father[a] += father[b];
father[b] = a;
}
else
{
father[b] += father[a];
father[a] = b;
}
}
int find(int i)
{
while (father[i] > 0) i = father[i];
return i;
}
原文地址：https://www.cnblogs.com/lxc1910/p/9690539.html