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
邻接表，每次记录与某个点有关的边的条数，然后标记这个点。最后判断是否是能有m条边就ok。
代码：
#include <bits/stdc++.h> using namespace std; int first[80000],nex[80000],u[80000],v[80000]; int n,m,k,nv,ve,visited[40001]; int main() { memset(first,-1,sizeof(first)); cin>>n>>m; for(int i = 0;i < m;i ++) { cin>>u[i]>>v[i]; nex[i] = first[u[i]]; first[u[i]] = i; u[i + m] = v[i]; v[i + m] = u[i]; nex[i + m] = first[u[i + m]]; first[u[i + m]] = i + m; } cin>>k; while(k --) { int c = 0; cin>>nv; for(int i = 0;i < nv;i ++) { cin>>ve; visited[ve] = k; int ver = first[ve]; while(ver != -1) { if(visited[v[ver]] != k) { c ++; } ver = nex[ver]; } } if(c == m)cout<<"Yes"<<endl; else cout<<"No"<<endl; } }