Compilare: user.cpp: In function ‘int main()’: user.cpp:34:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (i <= s.length() - 1) ~~^~~~~~~~~~~~~~~~~ user.cpp:41:108: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (s[i] != 'x' && s[i] != 'y' && s[i] != 'z' && s[i] != 't' && s[i] != '-' && s[i] != '+' && i <= s.length()-1) ~~^~~~~~~~~~~~~~~ user.cpp:56:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (i > s.length() - 1) i--; ~~^~~~~~~~~~~~~~~~ user.cpp:57:45: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (s[i] == '-' || s[i] == '+' || i == s.length()-1) ~~^~~~~~~~~~~~~~~