Airline Routes

Given a map of airline routes, you are supposed to check if a round trip can be planned between any pair of cities.

Input Specification:

Each input file contains one test case. For each case, the first line gives two positive integers N (2) and M (≤), which are the total number of cities (hence the cities are numbered from 1 to N) and the number of airline routes, respectively. Then M lines follow, each gives the information of a route in the format of the source city index first, and then the destination city index, separated by a space. It is guaranteed that the source is never the same as the destination.

After the map information, another positive integer K is given, which is the number of queries. Then K lines of queries follow, each contains a pair of distinct cities' indices.

Output Specification:

For each query, output in a line Yes if a round trip is possible, or No if not.

Sample Input:

12 19
3 4
1 3
12 11
5 9
6 2
3 2
10 7
9 1
7 12
2 4
9 5
2 6
12 4
11 10
4 8
8 12
11 8
12 7
1 5
20
11 4
12 7
3 6
2 3
5 3
3 9
4 3
8 3
8 10
10 11
7 8
7 1
9 5
1 9
2 6
3 1
3 12
7 3
6 9
6 8

 

Sample Output:

Yes
Yes
No
No
No
No
No
No
Yes
Yes
Yes
No
Yes
Yes
Yes
No
No
No
No
No

#include<bits/stdc++.h>
using namespace std;

vector<int> vc;
vector<set<int> > vd;
vector<pair<int,int> > v;
pair<int,int> p;
stack<int> s;
int ddfs,color;

void tarjan(int x)
{
    set<int>::iterator its;
    v[x].first=-(++ddfs);
    v[x].second=ddfs;
    s.push(x);
    for(its=vd[x].begin();its!=vd[x].end();++its)
    {
        if(!v[*its].first)
        {
            tarjan(*its);
            v[x].second=min(v[*its].second,v[x].second);
        }
        else if(v[*its].first<0)
        v[x].second=min(v[x].second,-v[*its].first);
    }
    if(v[x].first+v[x].second==0)
    {
        v[x].first=-v[x].first;
        vc[x]=++color;
        for(;s.top()!=x;s.pop())
        {
            vc[s.top()]=color;
            v[s.top()].first=-v[s.top()].first;
        }
        s.pop();
    }
}
int main()
{
//  ios::sync_with_stdio(false);
//  freopen("data.txt","r",stdin);
    int n,k,x,c1,c2;
    scanf("%d %d",&n,&k);
    v.resize(n,pair<int,int>(0,0));
    vd.resize(n);
    vc.resize(n);
      for(;k--;)
      {
        scanf("%d %d",&c1,&c2);
        c1--,c2--;
        vd[c1].insert(c2);
      }
      ddfs=0;
      color=0;
      for(int i=0;i<v.size();i++)
      {
          if(!v[i].first)
          tarjan(i);
    }
      scanf("%d",&k);
      for(;k--;)
      {
        scanf("%d %d",&c1,&c2);
        c1--,c2--;
        if(vc[c1]-vc[c2])
         printf("No\n");
        else
           printf("Yes\n");
      }
      return 0;
}