Compilare: main.cpp: In function 'bool is_digit(char)': main.cpp:18:21: warning: suggest parentheses around '&&' within '||' [-Wparentheses] return a >= 'a' && a <= 'z' || a <= 'Z' && a >= 'A'; ~~~~~~~~~^~~~~~~~~~~ main.cpp: In function 'int main()': main.cpp:23:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i < a.size(); ++i){ ~~^~~~~~~~~~