compare segmantation fault

Hi everyone.

I wrote my first useful program in C++ [maybe after hello world]. After hours of effort to make it survive the compilation [and it finally did] I can't make it work right. I get at runtime the message : Segmentation fault. using gdb I found that it is somehow related to the comparison functor I defined... still can't understand how can one create a segmentation fault without arrays.

I have included the parts of the code which I think that are relevant. If it helps , I am trying to implement Kruskal's algorithm to find minimal spanning tree:

Code:

//this is the functor:
struct lvert
{
  bool operator()(const vert &v1,const vert &v2)
  {
        //comparing the graph's verticles so they can be ordered.
    	if(v1.x==v2.x)
			return v1.y<v2.y;
		return v1.x<v2.x;
  }
};

//gdb says that the problem comes from here:
int unite(vert v1,vert v2,map<vert,set<vert,lvert> ,lvert > &dsets,vector<vector<vert> >& verts)
{
	cout << v1.x << ":" << v1.y << " " << v2.x << ":" << v2.y  << endl;
	set<vert,lvert> *S1 = &dsets[verts[v1.x][v1.y]]; //I guess this is where something is wrong
	set<vert,lvert> *S2 = &dsets[verts[v2.x][v2.y]]; //or maybe here?


	set<vert,lvert>::iterator i;
	for(i=S2->begin(); i!=S2->end(); i++)
	{
		
		verts[i->x][i->y] = verts[v1.x][v1.y];
		
	}
	S1->insert(S2->begin(),S2->end());
	dsets.erase(v2);

}


//and this code is somewhere in main() , it calls unite()
for(i=0; i<N; i++)
	{
		verts[i].resize(M);
		for(j=0; j<M; j++)
		{
			verts[i][j] = vert(i,j);
			dsets[vert(i,j)].insert(vert(i,j));
			cin >> state;
			vert v1(i,j);
			vert v2(i+1,j);
			vert v3(i,j+1);
			if(state==3)
			{
				Sedges.erase(edge(i,j,i,j+1));
				unite(v1,v3,dsets,verts);
				Sedges.erase(edge(i,j,i+1,j));
				unite(v1,v2,dsets,verts);
			}
			if(state==2)
			{
				Sedges.erase(edge(i,j,i,j+1));
				unite(v1,v3,dsets,verts);
			}
			if(state==1)
			{
				Sedges.erase(edge(i,j,i+1,j));
				unite(v1,v2,dsets,verts);
			}

			
		}
		
	}

bar.

Code:

Program received signal SIGSEGV, Segmentation fault.
0x0804a0b0 in lvert::operator() (this=0xbffff5d8, v1=@0x8050470, v2=@0x8)
    at p2.cpp:62
62              if(v1.x==v2.x)
(gdb) list
57
58      struct lvert
59      {
60        bool operator()(const vert &v1,const vert &v2)
61        {
62              if(v1.x==v2.x)
63                              return v1.y<v2.y;
64                      return v1.x<v2.x;
65        }
66      };
(gdb) back
#0  0x0804a0b0 in lvert::operator() (this=0xbffff5d8, v1=@0x8050470, v2=@0x8)
    at p2.cpp:62
#1  0x0804a532 in std::_Rb_tree<vert, std::pair<vert const, std::set<vert, lvert, std::allocator<vert> > >, std::_Select1st<std::pair<vert const, std::set<vert, lvert, std::allocator<vert> > > >, lvert, std::allocator<std::pair<vert const, std::set<vert, lvert, std::allocator<vert> > > > >::lower_bound (
    this=0xbffff5d0, __k=@0x8) at stl_tree.h:1323
#2  0x0804a4ad in std::map<vert, std::set<vert, lvert, std::allocator<vert> >, lvert, std::allocator<std::pair<vert const, std::set<vert, lvert, std::allocator<vert> > > > >::lower_bound (this=0xbffff5d0, __x=@0x8) at stl_map.h:508
#3  0x08049630 in std::map<vert, std::set<vert, lvert, std::allocator<vert> >, lvert, std::allocator<std::pair<vert const, std::set<vert, lvert, std::allocator<vert> > > > >::operator[] (this=0xbffff5d0, __k=@0x8) at stl_map.h:316
#4  0x08048a9f in unite (v1={x = 0, y = 1}, v2={x = 1, y = 1},
    dsets=@0xbffff5d0, verts=@0xbffff5c0) at p2.cpp:82
#5  0x080490b7 in main () at p2.cpp:155
(gdb)

That's what the debugger gives.

bar.