prepare (1/3): 
finished

make (2/3): 
g++ -O2 -Wall -Werror main.cpp -o main -std=c++11
main.cpp: In member function 'bool Graph::isDAG()':
main.cpp:50:31: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
             for (int j = 0; j < edges.at(i)->size(); ++j) {
                             ~~^~~~~~~~~~~~~~~~~~~~~
main.cpp: In member function 'int Graph::prim()':
main.cpp:84:31: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
             for (int j = 0; j < edges.at(i)->size(); ++j) {
                             ~~^~~~~~~~~~~~~~~~~~~~~
main.cpp:89:35: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
         while (used_vertex.size() < num_of_vertex) {
                ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
main.cpp:94:31: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
             for (int i = 0; i < used_vertex.size(); ++i) {
                             ~~^~~~~~~~~~~~~~~~~~~~
main.cpp:106:43: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
                         for (int j = 0; j < edges.at(i)->size(); ++j) {
                                         ~~^~~~~~~~~~~~~~~~~~~~~
main.cpp:117:43: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
                         for (int j = 0; j < edges.at(i)->size(); ++j) {
                                         ~~^~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
Makefile:2: recipe for target 'main' failed
make: *** [main] Error 1

build failed