Print adjacency list - c++11

Question

I've just started with graphs and was printing adjacency list using vector of pairs and unordered_map, though when I test my code against random custom inputs, it matches with the expected results but when I submit it to the online judge it gives me a segmentation fault.
Problem:
Given number of edges 'E' and vertices 'V' of a bidirectional graph. Your task is to build a graph through adjacency list and print the adjacency list for each vertex.
Input:
The first line of input is T denoting the number of testcases.Then first line of each of the T contains two positive integer V and E where 'V' is the number of vertex and 'E' is number of edges in graph. Next, 'E' line contains two positive numbers showing that there is an edge between these two vertex.
Output:
For each vertex, print their connected nodes in order you are pushing them inside the list .
#include<iostream>
#include<unordered_map>
#include<vector>
#include<algorithm>
using namespace std;
int main()
{
int T;
cin>>T;
while(T--){
int nv,ne;
cin>>nv>>ne;
vector<pair<int,int>> vect;
for(int i=0;i<ne;i++)
{ int k,l;
cin>>k>>l;vect.push_back( make_pair(k,l) );
}
unordered_map<int,vector<int>> umap;
for(int i=0;i<ne;i++)
{
umap[vect[i].first].push_back(vect[i].second);
umap[vect[i].second].push_back(vect[i].first);
}
for(int i=0;i<nv;i++)
{
sort(umap[i].begin(),umap[i].end());
}
int j=0;
for(int i=0;i<nv;i++)
{
cout<<i<<"->"<<" ";
for( j=0;j<umap[i].size()-1;j++)
{
cout<<umap[i][j]<<"->"<<" ";
}
cout<<umap[i][j];
cout<<"\n";
}
}
return 0;
}
Example:
Input:
1
5 7
0 1
0 4
1 2
1 3
1 4
2 3
3 4
Output:
0-> 1-> 4
1-> 0-> 2-> 3-> 4
2-> 1-> 3
3-> 1-> 2-> 4
4-> 0-> 1-> 3

Answer 1

"segmentation fault" is caused when a vertex has no edges. I do not see any constraints that all vertices have at least one edge in the problem description. For example, let's consider this input.
1
3 1
0 1
Here vertex 2 does not have any edges. Let's take a look what happens in the printing loop.
for(int i=0;i<nv;i++)
{
cout<<i<<"->"<<" ";
for( j=0;j<umap[i].size()-1;j++)
{
cout<<umap[i][j]<<"->"<<" ";
}
cout<<umap[i][j];
cout<<"\n";
}
umap[i].size()-1 is dangerous : As vector<T>::size() returns an unsigned integer. So if the size is 0 then it is 0-1 and that causes underflow
Even if the first was solved (something like (int)umap[i].size()-1), the following line cout<<umap[i][j]; will try to print umap[i][0] which is invalid if the size is 0
So I would change that code like:
for(int i=0;i<nv;i++)
{
cout<<i;
for( j=0;j<umap[i].size();j++)
{
cout<<"->"<<" "<<umap[i][j];
}
cout<<"\n";
}