P11049 说不能用 C++14(GCC9) 提交，我改用 C++14 就成了这样

No valid executable file was produced by the compiler
/tmp/compiler_dv0r1gpb/src: In function ‘std::vector<long long int> calculate_costs(std::vector<int>, std::vector<int>, std::vector<int>, std::vector<int>)’:
/tmp/compiler_dv0r1gpb/src:10:23: 警告：comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<int>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   10 |         for (int i=0;i<e.size();i++) {
      |                      ~^~~~~~~~~
/tmp/compiler_dv0r1gpb/src: In function ‘int main()’:
/tmp/compiler_dv0r1gpb/src:45:23: 警告：comparison of integer expressions of different signedness: ‘int’ and ‘std::vector<long long int>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   45 |         for (int i=0;i<res.size();i++) {
      |                      ~^~~~~~~~~~~
/nix/store/p58l5qmzifl20qmjs3xfpl01f0mqlza2-binutils-2.40/bin/ld: ./ccw4tJHd.o: in function `main':
src:(.text.startup+0x0): multiple definition of `main'; /tmp/compiler_dv0r1gpb/lib.o:src:(.text.startup+0x0): first defined here
collect2: 错误：ld 返回 1