DFS n-queens problem. Why my code is so slow? - c++

Question

Recently got an assignment on solving the n-queens problem using BFS/DFS.
My BFS stops at N=7 and I think it's understandable but DFS has a problem with N=8 and goes on for a few minutes.
Do you have any suggestions on how to speed it up?
Also, can you recommend what should I learn to speed up my code in general?
Here's my code:
#include <iostream>
#include <queue>
#include <vector>
#include <string>
#include <chrono>
#include <stack>
using namespace std;
const int N = 6;
int states_generated = 1;
void tree(vector<int> data) {
string arr[N][N];
for (int i = 0; i < N; i++) {
for (int j = 0; j < N; j++) {
arr[i][j] = " -";
}
}
for (int i = 0; i < data.size(); i++) {
arr[data[i] - 1][i] = " *";
}
cout << endl;
for (int i = 0; i < N; i++) {
for (int j = 0; j < N; j++) {
cout << arr[i][j];
}
cout << endl;
}
}
vector<int>* gen_v_variants(vector<int> parent) {
if (parent.size() == N) return nullptr;
vector<int>* vector_box = new vector<int>[N];
for (size_t i = 0; i < N; i++) {
vector_box[i] = parent;
vector_box[i].push_back(i + 1);
states_generated++;
}
return vector_box;
}
struct Node {
vector<int> queens;
bool checked = false;
queue<vector<int>> kids;
Node(vector<int> queens) {
this->queens = queens;
vector<int>* vector_box = gen_v_variants(queens);
if (vector_box != nullptr) {
for (int i = 0; i < N; i++) {
kids.push(vector_box[i]);
}
}
delete[] vector_box;
}
bool check() {
checked = (this->kids.size() == 0) ? true : false;
return checked;
}
~Node() {
}
};
bool validator(vector<int> data) {
for (int i = 0; i < N; i++) {
for (int j = i + 1; j < N; j++) {
if (data[i] == data[j] || abs(i - j) == abs(data[i] - data[j])) return false;
}
}
tree(data);
return true;
}
class DFS {
public:
int N;
stack<Node> st;
int states_checked = 0;
int n_queens_solved = 0;
DFS(int N, Node data) {
this->N = N;
this->st.push(data);
}
void begin() {
while (this->st.size() > 0) {
if (this->st.top().queens.size() < N && !this->st.top().check()) {
add_to_stack();
}
else if (this->st.top().queens.size() == N) {
if (validator(st.top().queens)) {
this->n_queens_solved++;
this->states_checked++;
}
else {
this->states_checked++;
}
this->st.top().checked = true;
st.pop();
}
else if (this->st.top().check()) {
st.pop();
}
}
result();
}
void add_to_stack() {
Node* parent = &this->st.top();
st.push(parent->kids.front());
parent->kids.pop();
parent = nullptr;
}
void result() {
cout << endl << "States: " << states_generated << "\nStates checked for n-queen problem: " << this->states_checked
<< "\nN-Queens problems solved: " << this->n_queens_solved << endl;
}
~DFS() {}
};
int main() {
using chrono::high_resolution_clock;
using chrono::duration_cast;
using chrono::duration;
using chrono::milliseconds;
auto t1 = high_resolution_clock::now();
vector<int> start;
Node a(start);
DFS test(N, a);
test.begin();
auto t2 = high_resolution_clock::now();
duration<double, milli> ms_double = t2 - t1;
cout << ms_double.count() / 1000 << "s";
}