foo.cc: In function 'int main()':
foo.cc:16:9: warning: unused variable 'res' [-Wunused-variable]
   16 |     int res;
      |         ^~~
foo.cc: In function 'int dfs(int, int)':
foo.cc:13:8: warning: control reaches end of non-void function [-Wreturn-type]
   13 |     dfs(i+1,j+1);//前i+1个坑中最后连续j+1个坑里都有物质，
      |     ~~~^~~~~~~~~