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

Recently got an assignment on solving n-queens problem using BFS/DFS.
My BFS stops at N=7 and I think it’s understandable but DFS has problem with N=8 and goes on for few minutes.

Do you have any suggestions 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";

}

Source: Windows Questions C++

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