dfs

#include <bits/stdc++.h>
using namespace std;

int arr[4] = { 1,2,3,4 };
int visited[4];

void dfs(vector<int>& v) {
    auto sz = v.size();
    if (sz < 4) {
        for (int i = 0; i < 4; ++i) {
            if (!visited[i]) {
                v.push_back(arr[i]);
                visited[i] = 1;

                dfs(v);
                v.pop_back();
                visited[i] = 0;
            }
        }
        return;
    }

    for (auto& c : v)
        cout << c << ' ';
    cout << '\n';
}

int main() {
    vector<int> v;
    dfs(v);

    return 0;
}