POJ 1611 The Suspects

The Suspects Time Limit: 1000MS   Memory Limit: 20000K Total Submissions: 32952   Accepted: 15977 Description Severe acute respiratory syndrome (SARS), an atypical pneumonia of unknown aetiology, was recognized as a global threat in mid-March 2003. To minimize transmission to others, the best strategy is to separate the suspects from others.  In the Not-Spreading-Your-Sickness University (NSYSU), there are many student groups. Students in the same group intercommunicate with each other frequently, and a student may join several groups. To prevent the possible transmissions of SARS, the NSYSU collects the member lists of all student groups, and makes the following rule in their standard operation procedure (SOP).  Once a member in a group is a suspect, all members in the group are suspects.  However, they find that it is not easy to identify all the suspects when a student is recognized as a suspect. Your job is to write a program which finds all the suspects. Input The input file contains several cases. Each test case begins with two integers n and m in a line, where n is the number of students, and m is the number of groups. You may assume that 0 < n <= 30000 and 0 <= m <= 500. Every student is numbered by a unique integer between 0 and n−1, and initially student 0 is recognized as a suspect in all the cases. This line is followed by m member lists of the groups, one line per group. Each line begins with an integer k by itself representing the number of members in the group. Following the number of members, there are k integers representing the students in this group. All the integers in a line are separated by at least one space.  A case with n = 0 and m = 0 indicates the end of the input, and need not be processed. Output For each case, output the number of suspects in one line. Sample Input 100 4 2 1 2 5 10 13 11 12 14 2 0 1 2 99 2 200 2 1 5 5 1 2 3 4 5 1 0 0 0 Sample Output 4 1 1 Source Asia Kaohsiung 2003

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比较明显的并查集，不断把小的一方作为源头合并然后遍历祖先为0的计数输出即可

• Source Code

  #include <cstdio>
  #include <cstring>
  #include <algorithm>
  using namespace std;
  int pre[30000+5];
  int root(int x)
  {
      while(x!=pre[x])
      {
          x=pre[x];
      }
      return x;
  }
  int main()
  {
      int n, m;
      while(~scanf("%d %d", &n, &m))
      {
          if(!n&&!m) break;
          for(int i=0;i<n;i++)
              pre[i]=i;
          while(m--)
          {
              int k, a[30000+5];
              scanf("%d", &k);
              for(int i=1;i<=k;i++)
                  scanf("%d", &a[i]);
              sort(a+1,a+k+1);
              for(int i=2;i<=k;i++)
              {
                  if(root(a[i])<root(a[1]))
                      pre[root(a[1])]=root(a[i]);
                  else pre[root(a[i])]=root(a[1]);
              }
          }
          int ans=0;
          for(int i=0;i<n;i++)
              if(root(i)==0) ans++;
          printf("%d\n", ans);
      }
      return 0;
  }