Compilare: main.cpp: In function 'bool check()': main.cpp:20:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < letters.size(); ++i) ~~^~~~~~~~~~~~~~~~ main.cpp: In function 'void backtrack(int)': main.cpp:34:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (depth == letters.size() - 1) ~~~~~~^~~~~~~~~~~~~~~~~~~~~ main.cpp: In function 'int main()': main.cpp:62:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int j = 0; j < a.size(); ++j) ~~^~~~~~~~~~ main.cpp:72:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int j = 0; j < a.size(); ++j) ~~^~~~~~~~~~