/in/foo.cc: In function 'void dfs1(int, int)':
/in/foo.cc:16:14: warning: iteration 19 invokes undefined behavior [-Waggressive-loop-optimizations]
      f[i][u] = f[i - 1][f[i - 1][u]];
      ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc:15:19: note: within this loop
  for(int i = 1; i <= 20; i++)
                 ~~^~~~~