Compilare: main.cpp: In function 'void build_graph()': main.cpp:82:25: warning: unused variable 'x' [-Wunused-variable] for (char i='a', j, x; i<='z'; ++i) { ^ main.cpp: In function 'void write()': main.cpp:115:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i=0; i<letter.size(); ++i, fout << ' ') ~^~~~~~~~~~~~~~