C++ input difficulty - c++

Question

I am trying to solve the following problem from ACM Timus:
http://acm.timus.ru/problem.aspx?space=1&num=1242 , but I don't how to read the input correctly. I am using two while loops. The first one terminates once a string is written in the input but the second one doesn't work after that. Here's my code:
#include <iostream>
#include <vector>
using namespace std;
struct Pair
{
void AddParent(int a)
{ parents.push_back(a); }
void AddChild(int a)
{ children.push_back(a);}
vector<int> parents;
vector<int> children;
};
vector<bool> visited;
vector<Pair> graph;
void DFS1(int n)
{
visited[n] = true;
for(int i = 0 ; i < graph[n].parents.size() ; i++)
{
int t = graph[n].parents[i];
if(!visited[t])
DFS1(t);
}
return;
}
void DFS2(int n)
{
visited[n] = true;
for(int i = 0 ; i < graph[n].children.size() ; i++)
{
int t = graph[n].children[i];
if(!visited[t])
DFS2(t);
}
return;
}
int main()
{
int n;
cin >> n;
graph.resize(n);
visited.resize(n);
int a,b,c;
vector<int> victim;
////////////////////////////////
while(cin >> a && cin >> b)
{ a--;b--;
graph[a].AddParent(b);
graph[b].AddChild(a);
}
cin.clear();
cin.ignore();
while(cin >> c)
{
victim.push_back(c);
}
////////////////////////////////
for(int i = 0 ; i < victim.size() ; i++)
if(!visited[victim[i]]){
DFS1(victim[i]);
DFS2(victim[i]);
}
bool vis = false;
for(int i = 0 ; i < n ; i++)
if(!visited[i])
{
vis = true;
cout << i + 1 << " ";
}
if(!vis)
cout << 0;
return 0;
}

Answer 1

Before going to the second while loop you should clear the input stream cin.
while(cin >> a && cin >> b)
{ a--;b--;
graph[a].AddParent(b);
graph[b].AddChild(a);
}
cin.clear();
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
while(cin >> c)
{
victim.push_back(c);
}
And include the header limits
Why would we call cin.clear() and cin.ignore() after reading input?