Solution of https://leetcode.com/problems/cracking-the-safe/

sol.cc

class Solution {
public:
    void extend(forward_list<int>::iterator curr, forward_list<int>& route, vector<bool>& was, const int& k, const int& top) {
        if(!was[*curr]) was[*curr] = true;
        int neigh;
        bool changed = true;
        while(changed) {
            changed = false;
            for(int i = 0; i < k; i++) {
                neigh = (*curr * 10) % top + i;
                if(was[neigh]) continue;
                was[neigh] = true;
                route.insert_after(curr, neigh);
                curr++;
                changed = true;
                break;
            }
        }
    }
    
    string crackSafe(int n, int k) {
        if(k == 1) {
            stringstream ss;
            ss << setw(n) << setfill('0') << 0;
            return ss.str();
        }
        int top = 1;
        for(int i = 0; i < n; i++) top *= 10;
        vector<bool> was(10000, false);
        forward_list<int> route{0};
        was[0] = true;
        for(auto it = route.begin(); it != route.end(); it++) extend(it, route, was, k, top);
        stringstream ss;
        for(const auto it : route) {
            if(it == 0) ss << setw(n) << setfill('0') << 0 << setw(1);
            else ss << it % 10;
        }
        return ss.str();
    }
};