1134 Vertex Cover （25 point(s)）

A vertex cover of a graph is a set of vertices such that each edge of the graph is incident to at least one vertex of the set. Now given a graph with several vertex sets, you are supposed to tell if each of them is a vertex cover or not.

Input Specification:

Each input file contains one test case. For each case, the first line gives two positive integers N and M (both no more than 104), being the total numbers of vertices and the edges, respectively. Then M lines follow, each describes an edge by giving the indices (from 0 to N−1) of the two ends of the edge.

After the graph, a positive integer K (≤ 100) is given, which is the number of queries. Then K lines of queries follow, each in the format:

Nv v[1] v[2]⋯v[Nv]

where Nv is the number of vertices in the set, and v[i]'s are the indices of the vertices.

Output Specification:

For each query, print in a line ​​Yes​​​ if the set is a vertex cover, or ​​No​​ if not.

Sample Input:

10 11
8 7
6 8
4 5
8 4
8 1
1 2
1 4
9 8
9 1
1 0
2 4
5
4 0 3 8 4
6 6 1 7 5 4 9
3 1 8 4
2 2 8
7 9 8 7 6 5 4 2

Sample Output:

No
Yes
Yes
No
No

经验总结：

 emmmm   其实就是求所给的点能否覆盖所有边，所谓覆盖的意思就是，这个顶点是一条边的其中一个顶点。问题很清楚，最终求得就是所有的边是否被访问到，所以输入时，每个顶点都存储自己可达的所有边的序号，然后根据所给的顶点集，遍历一遍所覆盖的边，最后判断所有的边是否被遍历完全即可~

AC代码

#include <cstdio>
#include <algorithm>
#include <vector>
#include <cstring>
using namespace std;
const int maxn=10010;
const int INF=0x3fffffff;
int n,m,k,a,b,t,Nv;
vector<int> adj[maxn];
bool flag[maxn]={false};
int main()
{
  scanf("%d%d",&n,&m);
  for(int i=0;i<m;++i)
  {
    scanf("%d%d",&a,&b);
    adj[a].push_back(i);
    adj[b].push_back(i);
  }
  scanf("%d",&k);
  for(int i=0;i<k;++i)
  {
    memset(flag,0,sizeof(flag));
    bool f=1;
    scanf("%d",&Nv);
    for(int j=0;j<Nv;++j)
    {
      scanf("%d",&t);
      for(int x=0;x<adj[t].size();++x)
          flag[adj[t][x]]=true;
    }
    for(int j=0;j<m;++j)
      if(flag[j]==false)
      {
        f=0;
        break;
      }
    printf("%s\n",f?"Yes":"No");
  }
  return 0;
}