Wrong Answer http://www.spoj.com/problems/WORDS1/ - c++

Question

I have written the following code for the above problem.
I have checked if Graph is connected by choosing the start node and doing DFS from it. And checking the conditions for Euler path and tour, in code that would be count= 2 or 0. It's working on all given test cases.But getting wrong answer on submission
#include<iostream>
#include<cmath>
#include<list>
#include<string>
#include<stack>
using namespace std;
class Graph
{
int V;
list<int> *adj;
int *in;
public:
Graph(int v)
{
this->V=v;
this->adj=new list<int>[V];
this->in=new int[V];
for(int i=0;i<V;i++)
{
in[i]=0;
}
}
void addEdge(int src,int des)
{
adj[src].push_back(des);
in[des]=in[des]+1;
}
void DFSUtil(bool visited[],int v)
{
stack<int> s;
s.push(v);
visited[v]=true; //mark v as visited;
while (!s.empty())
{
int top=s.top();
s.pop();
list<int> :: iterator it;
for(it=adj[top].begin();it!=adj[top].end();it++)
{
if(!visited[*it])
{
visited[*it]=true;
s.push(*it);
}
}
}
}
/*void DFSUtil(bool visited[],int v)
{
visited[v]=true;
list<int> :: iterator it;
for(it=adj[v].begin();it!=adj[v].end();it++)
{
if(!visited[*it])
{
DFSUtil(visited,*it);
}
}
}*/
// Graph reverseGraph()
// {
// Graph g(V);
// for(int i=0;i<V;i++)
// {
// list<int> :: iterator it;
// for(it=adj[i].begin();it!=adj[i].end();it++)
// {
// g.addEdge(*it,i);
// }
// }
// return g;
// }
bool isConnected()
{
//bool visited[V];
bool* visited=new bool[V];
for(int i=0;i<V;i++)
visited[i]=false;
int i=0;
int flag=0;
int n;
for(i=0;i<V;i++)
{
if(adj[i].size()>0)
{
n=i;
flag=1;
}
if(((int)adj[i].size()-in[i])==1 && in[i]==0) //selecting the start vertex i.e vertex with no incoming edges
{
n=i;
break;
}
}
if(i==V&&flag==0)
return 0;
DFSUtil(visited,n); //dfs to check if every node is reachable fro start vertex
for(int i=0;i<V;i++)
{
if(visited[i]==false && adj[i].size()>0)
return 0;
}
// Graph gr=reverseGraph();
// for(int i=0;i<V;i++)
// visited[i]=false;
// gr.DFSUtil(visited,n);
// for(int i=0;i<V;i++)
// {
// if(visited[i]==false && adj[i].size()>0)
// return 0;
// }
return 1;
}
bool isEuler()
{
int count=0;
int magnitude;
for(int i=0;i<V;i++) //check conditions on in and out edges, out edges=adj[].size
{
magnitude=in[i]-(int)adj[i].size();
if(magnitude<0)
magnitude=magnitude*-1;
if((magnitude)==1)
{
count=count+1;
}
else if(in[i]!=adj[i].size())
{
return 0;
}
}
if(count==1 || count>2)
return 0;
if(isConnected()==0) //check if graph is connected
return 0;
return 1;
}
};
int main()
{
int t;
//scanf("%d",&t);
cin>>t;
while(t--)
{
int n;
//scanf("%d",&n);
cin>>n;
string str;
if(n==1) //only one string entered
{
cin>>str;
cout<<"Ordering is possible.\n";
continue;
}
Graph g(26);
int src,des;
for(int i=0;i<n;i++)
{
cin>>str;
src=str[0]-'a';
des=str[str.length()-1]-'a';
g.addEdge(src,des);
}
if(g.isEuler())
{
cout<<"Ordering is possible.\n";
}
else
{
cout<<"The door cannot be opened.\n";
}
}
}

Answer 1

In your function isConnected, you attempt to find the start vertex with no incoming edges by breaking out of the for loop upon a certain condition. If this condition is not encountered, the loop variable i will contain the value 26, rather than an expected index in the range 0..25. When the value 26 is passed to the DFSUtil function it will caused an out of bounds array access. I get a crash on the line visited[v] = true;
A condition that will cause this to arise, is the test case from the linked site with the repeated word:
2
ok
ok
Your check to find the start vertex does not handle this case well.