func_code_string: #include <bits/stdc++.h>   #pragma GCC optimize( O3 )   using namespace std;   const int arr = 2e5 + 10;   const int ar = 2e3 + 10;   const long double pi = acos(-1);   const long double eps = 1e-10;   const long long md = 1e9 + 7;   vector<int> reb[(int)(3e5 + 10)];   bool use[(int)(3e5 + 10)];   int n, m;   int pred[(int)(3e5 + 10)];   vector<int> ans(0);   set<pair<int, int> > edges;   void upd_ans(vector<int> cur_ans) {     if (ans.empty() || (int)(cur_ans.size()) < (int)(ans.size())) {       ans = cur_ans;     }   }   void bfs() {     queue<int> q;     q.push(1);     use[1] = 1;     while (!q.empty()) {       int now = q.front();       q.pop();       for (auto wh : reb[now]) {         if (!use[wh]) {           use[wh] = 1;           q.push(wh);           pred[wh] = now;         }       }     }     if (use[n]) {       vector<int> cur_ans(0);       int cur = n;       while (cur != 1) {         cur_ans.push_back(cur);         cur = pred[cur];       }       cur_ans.push_back(1);       reverse(cur_ans.begin(), cur_ans.end());       upd_ans(cur_ans);     }   }   int num[(int)(3e5 + 10)];   vector<int> ways[(int)(3e5 + 10)];   int cnt;   void another_algo() {     int c1 = 0;     for (int i = 2; i <= n; i++) {       c1 += use[i];     }     if ((int)(reb[1].size()) != c1) {       for (int i = 1; i <= n; i++) {         if (edges.count({1, i})) {           for (auto j : reb[i]) {             if (j != 1 && !edges.count({1, j})) {               vector<int> cur_ans(0);               cur_ans.push_back(1);               cur_ans.push_back(i);               cur_ans.push_back(j);               cur_ans.push_back(1);               cur_ans.push_back(n);               upd_ans(cur_ans);               return;             }           }         }       }     } else {       for (int i = 2; i <= n; i++) {         if (use[i]) {           set<int> nums;           nums.clear();           for (auto wh : reb[i]) {             if (num[wh]) {               nums.insert(num[wh]);             }           }           if ((int)(nums.size()) == 0) {             num[i] = ++cnt;             ways[cnt].push_back(i);           } else if ((int)(nums.size()) == 1) {             int have = -1;             int not_have = -1;             for (auto wh : ways[*nums.begin()]) {               if (edges.count({i, wh})) {                 have = wh;               } else {                 not_have = wh;               }             }             if (not_have == -1) {               num[i] = *nums.begin();               ways[*nums.begin()].push_back(i);             } else {               int I = i;               int J = have;               int K = not_have;               vector<int> cur_ans(0);               cur_ans.push_back(1);               cur_ans.push_back(I);               cur_ans.push_back(J);               cur_ans.push_back(K);               cur_ans.push_back(I);               cur_ans.push_back(n);               upd_ans(cur_ans);               return;             }           } else {             int num1 = *nums.begin();             nums.erase(nums.begin());             int num2 = *nums.begin();             nums.erase(nums.begin());             int kek1 = -1;             int kek2 = -1;             for (auto wh : reb[i]) {               if (num[wh] == num1) {                 kek1 = wh;               }               if (num[wh] == num2) {                 kek2 = wh;               }             }             int I = kek1;             int J = i;             int K = kek2;             vector<int> cur_ans(0);             cur_ans.push_back(1);             cur_ans.push_back(I);             cur_ans.push_back(J);             cur_ans.push_back(K);             cur_ans.push_back(I);             cur_ans.push_back(n);             upd_ans(cur_ans);             return;           }         }       }     }   }   int main() {     scanf( %d%d , &n, &m);     for (int i = 0; i < m; i++) {       int u, v;       scanf( %d%d , &u, &v);       if (min(u, v) == 1 && max(u, v) == n) {         cout << 1 <<   n ;         cout << 1 <<     << n <<   n ;         return 0;       }       edges.insert({u, v});       edges.insert({v, u});       reb[u].push_back(v);       reb[v].push_back(u);     }     bfs();     if ((int)(ans.size()) <= 3 && !ans.empty()) {       cout << (int)(ans.size()) - 1 <<   n ;       for (auto i : ans) {         cout << i <<    ;       }       cout <<   n ;       return 0;     }     another_algo();     if (ans.empty()) {       cout << -1 <<   n ;     } else {       cout << (int)(ans.size()) - 1 <<   n ;       for (auto i : ans) {         cout << i <<    ;       }       cout <<   n ;     }   }   