foo.cc: In function 'void dfs(int)':
foo.cc:34:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < a[x].size(); i++) {
                  ~~^~~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:59:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s.length(); i++) {
                  ~~^~~~~~~~~~~~
foo.cc:45:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d %d", &x, &y);
   ~~~~~^~~~~~~~~~~~~~~~~

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