Dynamic Programming Solution for TSP in C++ - c++

Question

I have been trying to implement Dynamic Programming solution for TSP (Travelling Salesperson Problem) in C++. My code compiles but when I try to run the object file the program stops working and I am forced to close it.
Here is the code:
int tsp(std::vector<std::vector<int>> matrix) {
int n = matrix[0].size();
std::vector<std::vector<int>> A; // Vertex, Set-Size
std::set<int> S;
for(int i = 0; i < n; ++i) {
S.insert(i);
}
for(int i = 0; i < n; i++) {
if(S.size() == 2) {
A[i][2] = matrix[1][i];
}
else if(S.size() > 2) {
std::set<int>::iterator it;
for(it = S.begin(); it != S.end(); ++it) {
int s = S.size();
S.erase(i);
int sd = S.size();
int k = *it;
if((k != i) && (k != 1) && (A[i][s] > (matrix[k][i] + A[k][sd]))) {
A[i][s] = matrix[k][i] + A[k][sd];
}
}
}
}
return A[1][n];
}
Can someone please point out what mistake I am making.

Answer 1

You need to fill or resize a std::vector before calling operator[int] on it. A vector is basically an array that holds its size. So any out of bound access will cause a segmentation fault in runtime (if you are lucky) or will corrupt your memory.
You have a vector of vectors here, so you will need to iterate over two ranges and fill (or resize) vectors properly:
std::vector<std::vector<int>> A; // Vertex, Set-Size
for(int i=size; i>0; --i)
A.push_back(std::vector<int>);
for(int i=size; i>0; --i)
for(int j=size; j>0; --j)
A[i][j] = 0;
Even better:
A.resize(size);
for(auto& v : a) // (since you already have c++11)
v.resize(size, val); // fill with val