answer.code:4:7: error: expected nested-name-specifier before ‘ll’
    4 | using ll = long long;
      |       ^~
answer.code: In function ‘void solve()’:
answer.code:20:17: error: ‘function’ was not declared in this scope
   20 |                 function<int(int u, int c)> dfs = [&] (int u, int c) {
      |                 ^~~~~~~~
answer.code:20:17: note: ‘std::function’ is only available from C++11 onwards
answer.code:20:42: error: expression list treated as compound expression in functional cast [-fpermissive]
   20 |                 function<int(int u, int c)> dfs = [&] (int u, int c) {
      |                                          ^
answer.code:20:26: error: expected primary-expression before ‘int’
   20 |                 function<int(int u, int c)> dfs = [&] (int u, int c) {
      |                          ^~~
answer.code:30:25: error: ‘dfs’ was not declared in this scope; did you mean ‘ffs’?
   30 |                 cout << dfs(1, 1) << "\n";
      |                         ^~~
      |                         ffs