Compilare: user.cpp: In function ‘int main()’: user.cpp:23:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0 ,x; i < a.size(); ++i) ^ user.cpp:31:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0, x; i < a.size() - 1; ++i) ^ user.cpp:37:45: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = a.size() - 1, x; i < b.size(); ++i) ^ user.cpp: In function ‘int to_int(char)’: user.cpp:18:1: warning: control reaches end of non-void function [-Wreturn-type] } ^