supertrees.cpp: In function 'int construct(std::vector<std::vector<int> >)':
supertrees.cpp:9:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
9 | for (int i = 0; i < a.size(); i++) {
| ~~^~~~~~~~~~
supertrees.cpp:10:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
10 | for (int j = 0; j < a.size(); j++) {
| ~~^~~~~~~~~~
supertrees.cpp:16:16: error: void value not ignored as it ought to be
16 | return build(a);
| ~~~~~^~~