POJ-1611-The Suspects（并查集）

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

---

思路：并查集。

坑点：最后判断的时候，不能直接用pre[i]判断（即使压缩了也不行），因为如果编号节点A在后来又有了新父节点（编号），A的子节点还并没有改编号，这样pre[i]就不是真的find(i)了。

---

#include<cstdio>
using namespace std;
int n,m,k;
int pre[30005];
int find(int x){
    while(pre[x]!=x){
        int r=pre[x];
        pre[x]=pre[r];
        x=r;
    }
    return x;
}

void merge(int x,int y){
    int fx=find(x);
    int fy=find(y);
    if(fx!=fy){
        if(fx>fy) pre[fx]=fy;
        else pre[fy]=fx;
    }
}

int main(){
    int x,y;
    while(scanf("%d%d",&n,&m)){
        if(m==0&&n==0) return 0;
        for(int i=0;i<n;i++)
            pre[i]=i;
                
        while(m--){
            scanf("%d%d",&k,&x);
            k--;
            while(k--){
                scanf("%d",&y);
                merge(x,y); 
            }
        }
        
        int ans=1;        
        for(int i=1;i<n;i++)
            if(find(i)==find(0)) ans++;//这里不能直接pre[i]==0 ,因为可能编号节点会指向某节点，这时子节点还没有改编号。 
                            
        printf("%d
",ans);
    
    }
    return 0;
}